I'm a bit surprised with your decision to pull the rear bar. Where are you getting this advice from?
I run one of my Porsches without the rear bar, but I wouldn't expect it to help stabilize the rear in your situation.
Running no rear bar means running stiffer rear springs to maintain overall roll compliance. Stiffer rear springs mean you can't put power down as well, causing the back end to step under power.
The other thing I could imagine contributing to instability is mismatched shocks. How have you tuned the shocks so far? Have you used principles like these to guide your shock tuning?