Next time it starts to buck, tap the brake pedal just enough to light the brake lights. If the bucking stops, then I think it'* your torque lock sensor in the transmision. At least that is what I have learned about my wifes 2000 GP GT that does the same thing. Tranmisions starting around (1990?) were designed to lock at a certain speed, eliminating any slip that was in the prior designs. I think your sensor is just switching back and forth between lock and un-lock. It is designed to disengage whenever the brake is applied.
(Sorry for any mis-spelled words, I wish there a was spell check on here! )