[apalrd]

We had something similar last year, with each side slaved together in steering. (although we later separated the front and rear) We used Globe motors at around 3:1 or 2:1 reduction. In our testing, we initially used a simple proportional loop for our first test drives and it worked really well. We were using 5-turn pots, and had few problems with them.

How exactly are you relating joystick position to wheel position?

The window motors have thermal breakers in them, and will automatically shut down when they get too hot. That dosen't mean they are broken, it means you are overusing them.

Looking at the problems you found:
1. Do you mean the two drive modules are not turning to the same place? You need to set the centerpoint for your calculations for each side.

Also, if you mean they aren't turning the same way, all you can do is set the gains the same for both sides. If you have the same motors and the same sensors, they should need the same gains. We only calculated the gains for the left side of our robot, since the right side was a mirror of it.

2. If you aren't using I and D, set the P gain as high as it will go without excessive overshoot. IF you are using I and D, set the P gain so high it will oscillate. You probably don't need I and D unless you have problems with overshoot when the P gain is set reasonably.

3. Lower the P gain or add more I. If it twitches a bit, add D.