My brother's 87 vette will not stay running. It ran fine one night, the next day it runs terrible. I first thought it was a plug wire, we did a tune up, and no change. He has a moates adapter in his ECM because I had burned some tunes for him. I thought it was either the PROM on the adapter, or the flash chip I programmed for him. We put the OEM PROM in the car and no change. I threw the moates back in, but put a hypertech PROM on the maotes adapter, but backwards accidentally. The car run fine. Threw a code 41, but ran fine.

The car sounds like it is missing a cylinder when running rough. I am at a loss at what to do, because while it is running this bad it throws no codes. It runs this way in open loop, not sure about clsd loop as we have not been able to run the car that long.

Any advice about what would cause this? The fuel pressure is fine, new plugs, wires, cap and rotor. Someone suggested the ignition module, but it ran fine with I installed the moates adapter incorrectly.

Any suggestions would be greatly appreciated.
Craig