Ok fellas I've got an '02 4L60E with a TranGo Jr. shiftkit in it and a stock torque converter. I had a guy take a quick look at the car when I first got it all put together to see that it was running ok, and he told me it was running high line pressure. I'm not sure what the actual number was but I replaced the line pressure control solenoid and it didn't change anything.

When you put the car into drive you get a big WHAM, occasionally it will override the brakes a bit. Its not quite as bad in reverse and it does it the worst when its cold. Other than that initial engagement it drives great.

I've got the drivetrain pulled right now to fix an oil leak and I'm considering having someone look at it, but I'm not sure how evident the problem will be.

Sounds like the computer is in limp mode. This means it is has line pressure and will have 2nd & 3rd gears only. See what code it is throwing. Check to make sure that the wiring is hooked up correctly (no bent pins). If it was ok before the Shift kit, then this is most likely the problem here.

I purchased the trans rebuilt and the shift kit was in it prior to rebuild. However that does explain why the car would go 25mph on a flat without me even hitting the gas. I haven't had the chance to put it on the highway yet so I don't know if overdrive was kicking in or not. I will check into the limp mode though.

The funny thing is the computer did not throw any codes. I had my tuner hook up to it and he couldn't find anything wrong except the line pressure. However he is going to re-flash the ECM again to make sure nothing went wrong the first time.

Is the PCM tuned for the shift kit? It controls line pressure much like a shift kit and both together can cause problems

Re-flash did the trick, takes off much faster now that it has first gear to.