Miss after 5 min, runs great over 3000 RPM

[rollinsperf]

I'm working on Ali's 91 ZR1 that he brought to the shop after it sat for 3 years.
Runs fine for the 1st 5 minutes then runs like spark scatter in a distributor or a rev limiter until it hits 3000 and then it runs fine again to the red line.
Turns out he had FIC injectors put in 3 years ago, car ran fine for 2 weeks then same problem. He got mad at the car and parked it.
Replaced fuel, checked fuel pressure, ohm'd injectors, replaced them anyway with new units from FIC (the guys were were very helpful), replaced the coils with Delco units, replaced plug, ohm'd plug wires. I've had the plenum off so many times, I've got Jerry's Gaskets on speed dial.
I have no codes, and with all the new parts, the car runs fantastic, until it heats up, and then it runs fantastic over 3000 RPM. The problem occurs with the power switch on or off.
The only modification is a Prom for a 92 added after the 1st injector swap.
Typically our shop can fix anything, but this has us stumped.
Any suggestions?