My lil red express started having this problem where when you go to start it the second you turn the key to start it looses all power like someone disconnected the battery and it stays that way for a while until suddenly everything comes back on again by itself until the next time you turn the key to start. When you turn the key power lasts long enough to get a click from the starter relay and that's it.

You can disconnect the battery and reconnect and it doesn't change anything, I've checked for loose connections, tried wiggling wires, replaced the fusible link wire, tried using jumper cables to ground the frame directly and to go from positive to the starter stud on the starter relay where all the positive power goes back to on those trucks to make sure it wasn't some sort of hidden battery cable problem. I've also tried bypassing the amp meter and yesterday I spent more time on it running new wires to completely bypass the bulkhead connector all together and it still does the same thing.
I've gone through the wiring in the bulkhead connectors and all seems good. The last time the power was out I tried disconnecting the starter and alternator to see if something with those was causing it but it didn't help.

Has anyone ever had this problem and know what it was? I'm thinking it might be the ignition switch since that's the only thing that hasn't been touched now but there are some things like the headlights and cab lights that don't look like they go through the switch and they drop out too so I'd hate to spend the time digging into the column to replace it for nothing. Thanks

try a different battery first. Birdtracker

I forgot to mention, that was the first thing I tried before wasting time with everything else. Does the same thing also if it's boosted or hooked up to a large battery charger/booster.

Honestly sounds like a battery cable/terminal problem. Those cables corrode inside the terminals where you can't see it.

next thing is to try new positive and negative cables like the above post says. Positive to Starter relay and negative to block with another one to the frame and a smaller one to the body. Years back I had a truck that wouldn't run and it was the ground to cab. Birdtracker

I had an original battery cable with the fused positive starter eyelet and the small starter outlet surrounded by the plastic/rubber connector that failed one time. Turned out, that junction holding both wires just at the starter was bad. The large eyelet would not always have a connection. occasionally it worked, occasionally it didn't.
One day I just got so frustrated, I replaced the positive cable and never had a problem again. I could hook an ohm meter to both ends of the big wire, and if you moved the cable just right, it lost continuity, but most of the time it was good. Drove me crazy chasing that one.

Kind of reminded my of the no start coil wire terminal issue. There were times the truck simply would not start. We replaced everything on the ignition system several times, it would work for a month or so, then we started the entire process again.
One day a mechanic friend told me to give all the truck wiring on the ignition system "The tug test" Simply put, while each wire is connected to whatever it connects to, you grab the wire and give it a good tug. If it pulls apart, or fails, it was defective before and was probably at least part of your intermittent problem. In his words, you can't tug apart a good 22 gauge wire and it connector with your bare hand, you sure can't tug apart a 1416 or a 14 gauge wire and its connector apart. And if the terminal is secured with a nut on a threaded post, and it failed the tug test, it too was already defective. The threaded post, the material holding the threaded post, or the wire terminal should not pull loose if the connection is good or the housing is not cracked. The tug test is not a brutal assault, but a hook your fingers around the wire, tug on it, and see if it pulls free. I've found so many bad wire connectors with the simple pull test.
on the truck with the ignition problem, the tug test broke the switch source terminal on the coil positive, turned out, there was 2 or 3 strands of wire (16 gauge) still making contact inside that enclosed terminal end. The tug test broke those last 2 or 3 strands of wire. Crimped on a new terminal and never had a problem with that truck again.

Get a volt meter and figure out where you are loosing power. As said it sounds like bad battery terminal connection.

Check the grounded end of the ground cable to be sure it's on a clean surface.

or wait until dark and have your helper crank while you are under the hood looking for the power arc. if you see the arc, that is where the problem may be.

Thanks for the ideas.

Like I said originally the I've ruled out the battery cables and bypassed the bulkhead connector and ammeter and checked the various plugs and nothings made a difference.

I guess I'll go ahead and try replacing the ignition switch next, at the very least if that's not it I'll have a spare on hand.

Using jumper cables is not a proper test.

the two wires on the starter.....are they touching each other? saw a youtuber that had a car do the same thing, and that's what it was.

In this case it is... It looses all power and putting the jumper cables in the picture did not bring it back. But to please you I changed out battery cables and it made no difference.

Nope. But today I ruled out the ignition switch by disconnecting it and using a jumper wire at the plug and it still has the same problem BUT only when there is load from the starter on it. With the heavy cable to the starter disconnected everything works fine and if you disconnect the starter when the no power problem is happening it doesn't bring it back. So my next step is going to be trying to replace the starter relay and then throw a spare starter at it if that doesn't work. Something I would have tried already had the starter relay still been readily available locally.

The starter or relay will not cause a total loss of power in the cab. If you are losing power like that it is wiring related. Make a HD ground to the body from the battery and see what happens. Its a simple system and when acting up it should be diagnose-able in a few minutes.

This is your answer. Check voltage at the battery. "Click" the switch to get failure, then start following the circuit. Other end of the cable at the relay still have battery voltage? Power wire at the bulkhead have voltage? Etc. get the wiring diagram and trace the circuits and you can quit throwing parts at it. At some point you'll find where there ISN'T voltage. Don't forget that the problem could be on the ground side, so keep your meter grounded to the battery.

My brother-in-law had a FWD Charger that kept breaking clutch cables, like 3 or four in a couple of months. I was home on leave from the USAF and took a look at it. Opened the hood and asked him if the trouble started when he replaced the ground cable. He was lazy and grounded the battery to the body instead of the engine. It ran fine, but when the clutch was pushed in and the starter engaged, it was running all of the amps through the only piece of metal attached to both the engine and the body, the clutch cable. Heated the slug end at the fork up enough that it would snap. WIERD stuff can happen with bad grounds...