Car Bounces after 60 ft. What is causing this??

This car started off this summer running solid 10.2 @ 130 mph with a little instability early in the run. The car has 2.5" taller tires now, and hooks just fine, but after the 60 ft. it starts to bounce so much I have to let off the gas. After the bouncing stops I can get back into it and finish the run just fine. This is a foot brake car BTW. Someone suggested the chassis is unloading as the front end settles down. I really don't have a clue. We've tried Caltrac and rear shock adjustments and torsion bar adjustments with basically no difference. Pinion angle? Lower the rear end?