1990 Chevy Truck Engine stalls when let off throttle at hig

Both times it has happened I have taken it home and "pulled" the trouble codes out of the computer and all I have ever received is Code 12 - ECM/ Mil is working - (OBD I). When it happened the first time I thought it might have been a grounding issue/ battery Power issue and that I had lost power to the ECM.

Last night I "worked" the truck really hard. At full throttle - WOT - it downshifts and easily climbs to 100 + miles and hour - sorry have to quote everything in MPH - 90 model has no tach - and when off the throttle engine idles down, downshift of transmission is smooth - no loss of power - and rolls out to a smooth idle.

This condition of stalling only happens after an extended period of 70+ MPH - and then letting off the throttle

So - this weekend I am replacing the fuel filter and will try and check fuel pressure (if I can find right equipment, guage, etc)

I will replace ignition module and coil.

Lastly, I would like to get your thoughts on my exhaust. The truck has been re fitted with true dual exhaust. Each bank of cylinders leads to its own pipe to the rear of the truck. It is outfitted with three inch head pipes off of stock manifolds, dual high flow catalytic converters, dual flow master mufflers, and 2.5 inch tail pipes. The truck has always had a tendency - even though not setting trouble codes - and has two year old oxygen sensor - to run rich as evidenced by "black" inside the tailpipes.

Is it possible that I have catalytic converters that are severely flow restricted? Is it possible that the reduced exhaust flow could quench the fire in the cylinders at that high an RPM?

EGR valve appears to function normally - pushed up on the diaphragm from underneath and it appears to operate smoothly - also checked it with hand held vacuum pump and it moves as well. No trouble codes to indicate any problem with EGR Solenoid.

Ended up changing out the entire ignition including the distributor. As I dug further into it, I was only planning to change ignition module, but the pickup coil was much deteriorated - as was the physical structure of the distributor magnets etc - corroded and caked over with rust. Disassembled it, but made the decision to replace VS rebuild once I saw how bad it was.

New distributor, put aftermarket ignition module (Accel) in new "stock" distributor, and installed new Accel coil.

Also pulled fuel filter - interestingly - I found the paper element was loose inside the housing (probably manufacturers defect) - installed new filter. Did not check fuel pressure as I do not have the fittings to install behind the throttle body to put a gauge on the line. Even after replacing the filter and subsequent draining of the lines, the truck started normally - so I surmise the fuel pump is at least strong enough to refill the lines quickly.

Have not run the truck hard - giving time for new distributor to break in, but did notice that even at lower RPM is has a very fast drop off in idle under certain circumstances - not enough to stall the truck - just enough to notice - so I am thinking a check of the IAC motor is in store for this weekend.

I also think the truck idles to high just sitting stationary - but since this is computer controlled - I think that this might be IAC motor related as well.

One question - the plugs where the wiring harness plugs into my coil have the retention tabs broken off. Any way to fix this with replacement plugs? - Other than buying a new wiring harness?

Thanks for all the help, will keep you updated.