I wasn't meaning necessarily that the trans is locking up, just trying to clarify how it's behaving. Can you get more details, impressions, info, etc., on what's happening? Rpm, approx. throttle opening, is the car free to roll before the jerk (still could be locked but not likely), fluid level as mentioned, correct dipstick, make and type of shift kit installed, that sort of thing?
I had a switch-pitch set-up on my TH400 that allowed me to change the stall speed from about 1500 rpm to 2500 rpm at the flip of a switch, and the only difference was a bit higher rpm to get the car to move from a stop under normal driving, as well as the intended benefits of less load on the engine while idling in gear (041 cam) and the better launch when I got into it.