After watching the video, I would swear that you are running into the same issue that our bot was last year that I posted in this thread:
http://www.chiefdelphi.com/forums/sh...d.php?t=134694
Our issue is only when using PWM 0 (very bad) and 1 (not bad, but still present). Moving to 2 & 3, the issue cleared up. Our random fires were even in Disabled mode and sometimes lasting for 3 or 4 seconds at a time. Just for fun today, we tested the wiring this year using PWM 0 & 1 and the problem is still present with all new wiring/motors/controllers/code.
As a side, I'm assuming that your 4 motors are really 2 sets of 2 motors. Wouldn't you want to use a Y-cable on each pair so that they are getting the same signal at the same time and just calling them LEFT and RIGHT (or similar) in the code?