Hello everyone, this is my first post on here so bare with me please. My issue is on a 1998 Pontiac grand prix gtp. I found a 2002 gt for sale with a rebuilt motor and tranny but the car was junk, hit by a garbage truck on the street. I decided to drop the cradle of the gt and transplanted it into my gtp. While the motor was out, I top swapped the l36 and installed zzp 1.9 rockers and ls6 springs I believe from intense. I swapped this cradle into my gtp along with sd headers, an intense shift kit, and supporting items like plugs and thermostat. I sent an email to intense about getting my pcm tuned and just decided to pay the core charge and have them give me a tune. The car is all done now practically besides a vacuum leak which I cannot find as I've replaced all the lines on top of the blower and the vac line that zzp offers. The major problem now is the transmission oscillates between fourth and third at cruise and the car is sluggish after shifts then boosts back up after a couple seconds of lag at full throttle. The motor and transmission ran fine in the gt with no problems at cruising speeds. Can this be a tuning issue or electrical issue? Can this be from the shift kit? I have no idea at this point all replies can help, thank you.