[EricH]

Is is a 6WD "drop center" or "flat"? What motors are you using?

If you're running flat, get 4 omnis and put pairs in diagonal corners. This may help. (Why pairs? it smooths out the ride so you're replacing wheels less often.)

I'd guess that you're experiencing the "scrub" that causes many 4WD robots to bounce around and many >4WD robots to need omnis or a dropped set of wheels to turn.

If you're running one motor per side, you might add a motor to each side.