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I just had the TH400 behind my 454 rebuilt. They used a HP Performance kit and we upgraded the stall from stock (1500-1700) to 2000. Most verything works great: strong 2-3 upshifts, good downshifts, and good shift points. The only thing bothersome is the very light part throttle 1-2 upshift. It's very jarring, and almosts slams in gear. This is only at very light throttle, such as in stop-and-go traffice. When using moderate or WOT, the 1-2 shift is firm but in a good way. The light 1-2 feels heavy and awkward.

The vacuum modulator has been adjusted up and down and if it's set to shift early, the car almosts bucks at light part throttle, and if it's set to shift later, it really slams into gear. It doesn't chirp -- just slams. None of the other shifts chirp, but are all nice and firm.

What could be the cause of this? I'm taking the car back soon for routine post-build check but would like to be more informed on the problem. Any insights are appreciated.
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