undefined I've been plagued by this for almost a year. '97 Z28 6 spd, 34,000 miles. ATI Procharger 8 lb, twin intercooled; besides reprogrammed OBD I computer, ACCEL 300+ and plugs, engine is stock. Am now on 4th Optispark which now has about 350 (repeat 350) miles on it.

As of 2 weeks ago, my problems were (as reflected by the OBD II DTCs, because I put the OBD II back in to see if it made any difference. It didn't.):

1. Car ran normally at cruising under load or accelerating but would go to full rich (875 to 940 O2 millivolts on Scanmaster default display) at idle. Infrequently, the car would idle with normal random variation of O2 millivolts, but 95% of time would idle at full rich.

2. When cruising at steady load with normal random O2 millivolts, pressing down lightly on throttle would cause the engine to stumble like a flat spot; O2 reading would got to severe lean, e.g. 003 to 013 millivolts. Pressing further down on the throttle would break through the flat spot/stumble.

3. Car would occasionally stumble/cut out for a split second regardless of throttle position. Could happen repeatedly.

4. Running symptoms were the same with both the OBD I and OBD II computers. We disconnected all supercharger items and returned the car to stock (except for the plugs); no change in symptoms.

5. Car runs normally and strong at WOT with OBD I computer. (Cannot run at boost with OBD II computer as it isn't programmed for supercharger.)

6. OBD II DTCs were: 172 (rich bank 1), 175 (rich bank 2), 300 misfire, 1301 misfire, 0100 (MAF), 0400 (EGR), 420 (catalyst ineffective) and 430 (other catalyst ineffective). All of these did not happen all the time.

NOW, suddenly after about 350 miles with the OBD I back in and the new GM Optispark, I'm also showing DTCs 16 and 36 (low and high resolution signals for the Opti) on the Scanmaster. Car starting to run very rough. I'm back where I was months ago.

There aren't words to say how grateful I'd be to someone who can get me through this. Many thanks.