I have a 1994 corvette that I have been chasing a engine problem for a long time. It runs in open loop but when it gets warmed up and goes to closed loop it goes into a kind of limp mode. I have run real time data through the eehack program and found that the advanced timing is fine until acceleration, the advance spark goes to 1 deg and the car stumbles at best. It returns to normal but on acceleration does the same thing. What can cause this sudden drop in degree of timing on acceleration? All the other sensors seem to respond normally.