What rear springs are you using?
If your car is going straight. You might try turning up your shock setting in the rear. Your motor is putting out good HP, you may be hitting your rear shocks hard enuf that theyre going down to where theyre bottoming out or to a point you cant hold your suspension there, allowing the rear end to unload as it comes back up. This sometimes called basketballing. Believe it or not with big HP you can actually hit your suspension to hard. Try this approach before you go moving your ladder bar setting.