Code 42

[SoCalSlaughter]

Stuck! Ive had a code 42 on my 1990 Suburban 5.7 4X4 for about 2 months now. Cold start the truck will run rough, once warmed up it will run good. If im at a stop light for a few minutes the check engine light will come on and the rpm will drop, but the truck will take off fine. Ive tested the ignition module, checked with another unit. Swapped ecu, same result, swapped whole distributor and same result. Pulled whole wiring harness out, checked all wires and grounds. Only thing I havent changed is the MSD ignition coil. Its about 3 years old. Oh and havent change the PROM in the ecu. Check the set timing connector and is all good. I cannot for the life of me figure this one out. I have an idea its the PROM because the last owner said it had a short in the ECM-B circuit for about a year and would blow the fuse and shut the truck off. So im thinking maybe the PROM is bad from being blown so many times within that year. That circuit is all fixed but it has the same PROM. Any ideas? tests I can do?