do you have any diag equipment? GOOD fuel pressure gauge, scanner, inj tester?

if not, i would bring it in to a GOOD drivability shop. could be alot of things, but i would have too look at serial data and other things before i could accurately diagnose it.....

O2 sensor isnt going to tell the ECU exhaust temps, and if i recall, the 94 3.1 was a speed density system. GM had MAJOR issues with MAF's being bad from the factory in the late 80's, and went to a speed density (read: TPS and MAP based fuel metering)