Does the Soarer have an independant rear end? I notice this effect (stuttering and jumping) quite badly on my 1jz powered MA70 Supra - it has new shocks and pretty stock ride height.
I also noticed that Martin Donnon once wrote about it in HPI about his Soarer and polyurethane diff mounts/bushes didn't help at all.
The only thing I can advise is to launch at a higher RPM (seems to work in the Supra) - then it smoothly spins both rears without jumping around, but this does mean you lose some traction....and only regain it mid-way through 2nd gear!