[philso]

An easy way to test what Al is suggesting is to put some blocks under your robot so that none of the wheels touch the ground then "drive" the same manouvers that cause the problem. You now have no traction so none of the motors should stall due to the choice of wheel tread. You may also find certain wheels that don't turn, or do not turn smoothly, due to high friction in gearboxes or some other issue.

Good luck.

Phil