I bet you have a bad plug or coil. There can't be anything wrong with the trans to make it shift like that. To go from 3rd to 4th the forward clutch releases and the OD band applies. It can't happen fast enough back and forth to give a bucking sensation. You prob have a plug that has carbon tracks on the outside and when conditions are right the spark jumps to ground outside the plug instead of through the plug. Pull each coil and closely inspect inside each boot for the tracking.