My gut is still telling me there is an issue with the alternator. But I had it tested twice by 2 different shops and they said it was good. The unit is a Powermaster 90 amp built out of the old round back case. A buddy had one on his car that failed him on the road 3 times and the alternator still tested good. He replaced the alternator with a new one and never had a problem again. I believe the spike fried the tachometer.

The FSM has some pretty decent alternator T/S guides and rebuilding your own isn't too bad.

Parts source

You might be having an intermittent full field going on, does the ammeter peg out at all?

I agree with Sniper.
The ammeter will often reveal alot.
It must be observed in conjection with the situation as it reacts to current flowing.

Having a handheld voltmeter will also help considerably.
Measurements from it are also situational. For example, measuring the voltage at the alternator output stud ( to ground ) at slow idle might be different than fast idle or higher rpm.

Using the ammeter together with voltmeter readings can be very powerful combination for troubleshooting.

I don't know what Powermaster has done to get a roundback to produce 90 amps,
Checking the basics of any alternator are pretty easy. Opening up a roundback one must be a little more careful than a squareback, but neither are difficult. Keep the stator together with the rectifiers and nothing will get broken. Leave the pulley on. No reason to remove it unless the rotor or the bearings on that end need replacing.

Concur. Must be some super duper stator.

OE alternators were typically 37amp, 50/60 amp for A/C cars. When I rebuilt my 70, I replaced the OE stator with a 50amp NOS (to help supply a few more gizmos in my RR).

Everything seems to point to an intermittent alternator issue. Try another reman unit.