Pontiac Repair: 1992 Pontiac Sunbird, torque converter clutch, driving a stick shift

Question
Hello there,
I recently bought a 92 Pontiac Sunbird with the 3.1 multiport engine, V6. Since i bought it I've completed some regular maintenance such as oil change and tune up. The car ran fine before the tune up just started really hard. However, since I've done the tune up the car seems like it's been sputtering going 40 or 50 or 60. If i hit the gas, the car goes through all these speeds and runs fine. If i'm going down a road at exactly 40 mph the car starts shaking and stuttering. I have not gotten a check engine light or anything that may tell me something is wrong. Seeing how i don't know when any of the sensors were changed I was planning on changing the IAC, the MAP, and the o2 sensor as well but will this help me? What's causing the car to stutter at those 3 speeds only? Please let me know what you think it may be and if i should change those sensors too. Thank you for your time.

Answer
Hey Moe:

Well first off lets back up.. It ran fine before the tune up and now it doesn't??? Maybe one of the parts you put in caused the problem. The car is misfiring either from a dead cylinder or a lean misfire. When the car is warmed up and it hits 3'rd gear there is a device inside the transmission called TCC Torque converter clutch when you are cruising at a steady speed in 3'rd gear the TCC is applied and that misfire is felt more. The torque converter is not 100% effecent in transfering power from the engine to the transmission and the TCC "locks" the engine and transmission together so if the engine is not runing right you will certainly feel it with the TCC enguaged. It's like driving a stick shift car in a higher gear than you need for the road speed it lugs the engine same feel... Now back to your issue. What type of spark plugs did you put in this? Use only AC Delco and make sure they are gapped correctly. Do not use any magic splitfire/double platinum Etc Plugs. use one origianl equipment. How about plug wires Don't skimp on them either use only good quaility parts.. I would look at what you did first instead of looking for another problem.. Chnaginr an IAC will not fix a running down the road issue. The Map may be okay but it may be reading wrong due to the way the engine is running.. Chnaging the O2 sensor will not fix it either but if the car has a lot of miles on it you may want to chnage it anyways they do wear out over time they become "lazy" and do not produce enough voltyage or what is called cross counts. If you replace it only use a factory part also No aftermarket parts. Hope this helps you out a bit and good luck :}