So I've almost given up on trying to figure out the cause of this issue. While driving on a cold engine (below operating temp) I have to be very smooth with the throttle, otherwise my wideband shoots down to the rich end of the gauge (9, maybe 10:1) for a couple of seconds, the engine occasionally stumbles, and then the afr comes back up. Car starts and idles fine. Afr's at operating temp are perfect. Definitely seems to disappear once the car reaches a certain temp. I have no codes, I've tested all my sensors according to the tsrm, and I cannot find anything wrong. I know our cars are notorious for running rich, but this just won't stop bugging me. Car is a '90 turbo 5 speed with a 3" turbo back & apexi intake.