https://www.youtube.com/watch?v=l5yW...ature=youtu.be
Our robot was experiencing major drive issues. At full speed, it would veer significantly off to the side when both sides were driven at full power. At other times one side would seemingly stall.
I found a major problem was the low speed gear in the shifter gearbox. The bearing seat is approximately 20 thousands off center. This causes the gearbox to bind up. At full power, the motors can overcome this binding, resulting in one side going a bit slower- causing the veer. At low speeds, the motors wouldn't have enough momentum to overcome the binding, causing one side of the robot to lock up.
Here you can see it visibly off center.
https://www.youtube.com/watch?v=xBqy...ature=youtu.be
If you are having drive problems and nothing else seems like its wrong, check the gears. In our case, the offending gear is visibly off center. I hope this helps someone.