Out of the blue, my '99 FRC started having some sort of an electrical problem. The car will idle and generally drive fine, but it won't do so smoothly and quite a bit of power is missing (it needs almost 50kpa of MAP to idle).

When it first happened it was very intermittent. After about an hour of driving, the "REDUCED ENGINE POWER" message came up on the DIC, but it hasn't come back since.

The only PCM code thrown is P1571, which is an error on the signal that the traction control computer uses to request less power from the PCM. Thinking this was the issue, I disconnected the Trac/ABS computer and tested the line. It reads 5v at idle, just like the manual says it should. Leaving the Trac/ABS computer disconnected has no effect on the problem, so I don't think this could be the cause (my Trac computer has always been a bit iffy anyways, but I've never been a fan of driver aids).

I've logged everything I can think of with HP Tuners. The only anomaly I've seen is at WOT, bank 2 (I think this is the passenger-side bank?) O2s are very lean, in the 200mV range (bank 1 is normal, 900mV). There is no knock, no spark retard, no torque management, or anything else. LT and ST FTs are normal on both banks (so O2s are fine at part-throttle), as are injector duty cycles and spark advance. Throttle position seems to follow the movement of the gas pedal.

I've cleaned and checked every ground in the engine bay. I've also jiggled connectors and harnesses to see if I could affect the symptoms at idle. No luck. All cylinders have spark, and the plugs on bank 2 look normal.

Now the problem is no longer intermittent; the car always feels like its stumbling. Every few minutes, the car will "pop" out the exhaust.

The only thing I can think of is that the Traction Control Torque Request Circuit is being read incorrectly by the PCM; i.e. the PCM itself is bad. Is there any way to log (via HP Tuners, or whatever) the amount of power reduction the PCM thinks the Trac computer is requesting?

Any help is greatly appreciated!