i had this same prob. when i installed my jeep 60mm t.b what i found it to be was the car was not comming down to idle rpms fast enough so when the trans would shift into first gear the rpms would still be up a couple hundred and it would buck the car, check for some kind of vaccum leaks or you IAC

Mods:
CAI, OBX exhaust, Grillcraft grills, altezzas, ard ryver g6 17's wrapped w/ falken's, VIS racing c.f hood, 2 12" kicker comp's w/pioneer h.u, streetglow blue undercars, red dash kit, indiglo guages, n other shit