I think you need a new baseline from which to start tuning. I'll offer mine, since it was working very consistently on the same size tires:
- Pinion angle 3.5 degrees
- Link bar in lower hole
- 1/4 turn preload on driver side; 1/3 additional turn preload on passenger side (measured w/ me in the driver's seat)
- Rear shocks on middle extension setting
- Front shocks on lightest extension setting (if adjustable)
- Rear tires 17.5 psi on 8" rims
- Launch from 1700 rpm (gets it above idle, but not so high that the chassis starts to torque up)

Good luck!