[The Lucas]

As Al mentioned the chippies are designed for max power at high rpm. The chippies seem to lose power proportionally more than other motors as PWM values approach 127. In our 4-motor drive (drills and chippies) we noticed that at slow speeds (PWM values slightly out of deadzone) the the chippies would cut out and drills would continue to run. I think this is related to the easiness to backdrive the chippies, so it drives forward during a pulse and backdrives between pulses (or the pulse is not long enough to get out of stall). We corrected this in programing by giving the chippies a different PWM value curve at lower PWM values. Check the PWM values as supplied to the motors with DEBUG statements as previously mentioned.
Also check ur battery voltage. It may decrease significantly decrease during a match, draining ur power, especially if u have a 6:1 gear ratio. Last year when when MOEhawk was direct driving HI gear drills with chippies to match (we could drag race to goals in like 2 sec), the voltage dropped below 6v during the mad dash to the goals (it sent the controller into safe mode). After that dash the voltage would rise again but would still be a couple volts below starting voltage. If that is the case, I suggest u gear down like we did last year.