I have a Th350 that was given to me by a friend, he told me it would not stall and at the line it was very hard to hold. I thought "OH its a converter" I put a new 3500 converter in and put it in my car...YEP!! same thing! it surges forward and dont want to hold...if I grab the brakes hard it will barley hold...then launches like a rocket and shifts great....what can it be?  

Probably the shift kit in the valvebody.