[Joseph F]

We had a problem where our drive would stop on the ground and run fine on the table. It turned out that the set screws were unscrewing themselves so that the friction between the floor and the wheels was enough to cause the coupling to release the shaft from the motor and allow it to spin seperately. When up on a box or the table there was no friction (or significantly less at least) so that the set screws were still biting enough to turn the wheels. We also have had trouble due to loose chains and misaligned or tilted sprokets. If none of that works try changing the pwm cables, they may be bad or very loose.