1991 RMW ignition module woes

First post! I've got a 1991 RMW that got a replacement distributor due to bad bushings in the original unit. I noticed that after I replaced the distributor, the tachometer would jump around at light throttle cruise and the engine would stutter ever so slightly. It did not act like that before. The distributor was a cheap, parts store brand replacement unit that came complete with cap, rotor, and ignition module. To try to diagnose the issue, I first put the original cap and rotor on the new distributor and there was no change. I then swapped out the ignition module with the original factory module and the problem went away. I put the new cap and rotor back on and the problem was still gone. Great! Problem solved. Fast forward to this past summer, the car will try to stall out when it gets SUPER hot under the hood, usually waiting in busy drive-through lines or at long red lights when it's over 90ish degrees outside. I was able to reproduce the problem in my driveway and found it was a lack of spark. I replaced the ignition coil, no change. I've had TBI GM trucks do this to me when the ignition module was going bad. I ordered a generic, but quality, ignition module and my stutter/erratic tachometer problem returned immediately. I then ordered a factory replacement ACDelco ignition module and installed it. Same deal, the tachometer jumped around and the engine stuttered at light throttle. If I put the original 1991 ignition module back in, the problem goes away, except for the stalling in the summertime. Has anyone run into this problem before? Does everybody, including ACDelco, get their modules made by the same company nowadays? Do all new modules have some internal update that causes this issue? It seems odd that the only thing that behaves itself is the original ignition module. I doubt it's a grounding problem or a wiring problem, seeing as swapping out ignition modules changes the behavior.

I know it may seem dumb but have you disconnected the battery when switching out the modules? and have you made sure to have good thermal paste one the under side of the modules when you've installed them?

had my 89 crew cab acting up when it got hot under the hood similar to some of what your experiencing, but I replaced the factory ignition module with an accel one, disconnected the battery, reconnected it and the truck ran fine.

What you have is a heat related issue (besides the aftermarket ignition modules). Have you swapped either/or the IAT or water temp sensors? There is a spark control module on the passenger's side on the relay bracket, and beside the AC accumulator. I have not heard of them going bad, but it is possible. I had hot start issues with both my OCCs, and have never fully diagnosed them. Clear any codes in the computer while you are at it. The fuse for the computer/fuel pump is the small fuse container on the relay bracket on the passenger's side. If you remove the fuse for at least 2 minutes, the computer codes will be cleared.

I had a hot start issue with my 91 OCC. I was advised at the Wagonfest that it could be the starter. I changed the starter and the problem went away. I know this reply has nothing to do with the ignition module, but the issues may be unrelated.

You might want to reestablish the ground at the thermostat housing. It controls the computer, O2 sensor, and one other thing that I forget.

Oil pressure sensor

Try the ground at the thermostat housing. It is the ground for the computer, and O2 sensor. Does your car throw any codes?