Engine Starts Cutting Out & Back-firing In Drive, But Does Ok In 1st & 2nd Gear. Still Cuts Out A Little. On The Freeway, In Drive Or Over-drive, It Starts Back-firing And Sputtering, Loses Power And Then Re-engages, But Only For A Moment Or 2. Have Changed The Fuel Filter, The Tps And The K& N Filter. No Change In Performance. Have Checked Spark Plugs-they R Ok, Have Not Yet Replaced The Plug Wires, Anyone With Any Suggestions, Please Help !!!!!

Replace your pigtail harness to your optispark. I have had two of these cars and both had the same problem. if that doesnt work check the optispark itself. but if i were a betting man i would say that is your problem. about $50 for the pigtail from GM if i remember correctly but that has been a few years ago

they tend to tarnish on the inside of the casing for some reason. this problem drove me and the dealership nuts untill i finally discovered this issue. I swear i will never take my car back to a dealership again. Most are just parts changers..most and they rely way too much on that little computer of theirs.

I would check a little more basic. Vacuum leak around the intake.

Does the car run better first thing in the morning when started and running on cold idle? And then declines quickly as it warms up? If so, look for a vacuum leak.

If there are no breaks in the insulation of the spark plug wires, then check the resistance of each wire to see if one or more is bad. If they all check out, move on to another area or system.

If you have access to an oscilloscope you can check the pattern of each sensor on the car. I'm not sure if the correct patterns are on the web somewhere, but I took a class many moons ago on testing sensors which detailed out what different sensor patterns should look like.

Hope this helps.