My RR was always started after a week or two with a 5 sec crank, pump gas 3 times, crank again and good to go. But lately i crank it for 5 sec about 4 or 5 times with no start i know its getting fuel then after the 4th or 5th crank as i release the key i feel a shutter or something like the ignition got a spark, then boom fires right up and runs fine this seems to only happen when cold and sitting more than a few days. what could this be? what is going wrong? Thanks

Is this an MSD system? Anyway it sounds like you're not getting enough current/voltage to your distributor/coil/ignition until you release the starter which is pulling it all. An MSD needs close to 12v to work, thats why you wire a 12v supply directly to it. Interesting problem. I've seen cars start on the starter and then die, but never the other way around.

Makke sure that you have a fully charged battery.

its a chrome ecu with blaster II coil and MP dist, and a msd .8 ohm b/R. always runs fine, just weird how it doesn't lite till i feel the "shudder" then bang starts up.

After the shutter i turn the key again and it starts. Maybe ign switch?

Make sure the wires going to the ballast resistor is not loose!
Had the same thing happen to me when it stumble,it shook the car enough to get contact and start!
Make sure the metal contacts are on the terminal even if ya have to pull the plastic insulation back some!

VERY INTERESTING: My 528 hemi has started doing the same thing. Cranks over and when you let off the key it will fire. Sometimes start right up when you let off the key. I have the summit street strip digital ignition and msd pro billet distributor. It worked fine until recently.
Also, my battery is trunk mounted.

Double check the gap of the reluctor inside the distributor..
Use a brass feeler guage should be 0.006-0.008".

Since this is a passive part if the gap is too wide the ECU will not fire the coil.

Just my $0.00...

Took some quick voltage readings, the batt had 12.7v, voltage at B/R blue wire side was only 10.4v, and at the coil term was at 4.3v and slowly climbing this was with the key at run position. but the car did fire right up today, like i said it sems to do this after several days of sitting, Shouldn't i see more like 12v at the resistor? and 8 at the coil well back to watching the kids

A few days of sitting and your battery will go down a tad. Lower the readings where you checked as a result.

i agree the batt up until recently would hold 12.7 to 12.6 for weeks on end, but maybe its going bad. i don't like the 10.4v reading at the ballast, everything i read says it should have 12v, so have to go through wiring to see if i can get 12v .

Had the same problem with my buddy's 70 cuda.It was losing IGN 'run' (blue wire) during cranking.With the IGN.switch in the "run" position,check the voltage at the red wire (batt.)and the blue one,coming from the back of the switch. It should be within 1/2 volt of each other.Then disconnect your yellow wire on the starter relay,so it doesn't effect your readings,and check it again in the "start" position I don't know what to recomend,as I replaced it with a new switch from NAPA and had the same problem 1 month,150 miles later

Go back to basics: right from the gitgo I'd have someone turn the key as you have the coil to dist wire in hand held 1/4" from metal to confirm/deny that there is no spark then start checking voltages and subsystems to find the open starting w the pickup as Sinitro said. EDIT reread your post and yes your voltage drops are way too much

If you have a spare ignition box, install it and see what happens. A new box solved the same problem on my car.

if it only happens after sitting for days are you sure it isn't fuel related? like maybe all the cranking is needed to fill the carb bowls then it fires when it has fuel. just another thing to check.

well after more digging it looks like the MSD coil might be the problem with it hooked up it was getting about 5.6v replaced with an old coil i had on the shelf and it gets 7.7 volts, only other oddity is with the v/r hooked up ballast gets 10.5 volts, with it unpluged it gets 11.5v, this is a replacement Solid state type, maybe i'll toss the old mechanical style on and see what happens. This MSD coil would always be to hot to touch even hotter than the intake manifold. it was in the stock BB location.