Mine does the same thing. Is your seat loose? Tighten the nuts under car. I believe you have to tighten them to the same torque or you will pull one corner down farther the other.
500 hp supercharged 306 with a/c
New rebuild ,main support,custom ground cam,3 stage port Windsor Jr heads,3 stage port gt 40 intake,30# injectors,255lph fuel pump,70mm throttle body,70 mass air,power pipe,k&n filter, Vortec blower,12# pulley,MSD ignition.Pre 93 computer,
Suspension-90/10 struts,mega bite Jr's,drag launch kit,polyurethane bushings.
Drive train-center force clutch,adjustable clutch cable,aluminum drive shaft,safety loop,pro auburn,hardened axles,4:10 gears,3 inch studs