I have another answer to some of this. I know the theas old but the fix may remain the same. When I put the new motor in I sold my manifold vaccume lines with it and the guy who bought it let me know there was a small hole in the line going to the back of the intake under the foam covering. He said it was an easy fix but wanted to let me know so I'd say when its stumbles at tip in and lower rpms when you stab the gas it may be this. I looked forever on those lines but did not want to "ruin" it by actually removing the covering to verify no leaks. May have saved me tons of needless repairs and confusion. Id say check the MAF then these vac lines, they are tucked way back therebut take your time.

 

I GOT A 98 LS1 T/A, WITH ROUGH IDLING AND AT HIGH RPMs, I BOUGHT ABOUT A YEAR AGO IT HAS A MILD CAM, NOS, 373 GEARS, TUNE, HEADERS,WELL ANYWAYS I WAS DRIVING HOME AND STOPPED AT A STORE TO BUY SOMETHING THE CAR WAS RUNNING GOOD AND ALL OF A SUDDEN IT RUNS LIKE CRAP, REPLACED SPARK PLUGS,FUEL FILTER, 1 COIL ALL 4 O2 SENSORS, IT GAVE ME BAD O2 SENSORS (STILL) I BOUGHT IT WITH EVERYTHING ABOVE EXCEPT HOW IT RUNS, I THINK THE OWNER WHO HAD IT, DID NOT DO A GOOD TUNE, IT FEELS LIKE A MISFIRE, AND EXHAUST SMELLS LIKE UNBURNT FUEL, TOOK IT TO A MECHANIC AND HE PUT THE TEMP GUN AND SAID IT MIGHT BE LOOSING COMPRESSION BECAUSE SOME CYL. WERE COLD AS COMPARED TO OTHERS. SO I MIGHT REPLACE ALL EIGHT IGNITION COILS, HELP ANYONE. WHAT IS THE OPTI SPARK AND DOES MY LS1 HAVE IT?