I moved this from the Official forums because they are not going to respond until January 5th (they are on a break).
Now for the problem, your motors are overheating and tripping their PTC’s. This video should explain it:
JPearman can also elaborate on all the details
Are they geared for speed or torque? It could be that there is too much load on your drive motors and late in the day they are getting overheated.
Also, it is possible for only your drive to stop, but your lift still work fine. There are two reasons for this.
If you are tripping the internal motor PTC, then only the motors being overworked will stop.
Or, you may be tripping the PTC’s in the Cortex. There is one PTC for ports 1-5, and another for ports 6-10. Therefore it is possible to lose power to all ports 1-5 OR 6-10 at the same time.
If you are tripping the PTC’s in the Cortex, but only in later matches, you can get some coolant spray for your motors. The best solution, of course, is to let your motors cool off if you have time, so use the spray sparingly and as a last resort.
From my response in the official Q&A:
It sounds like you are tripping the breakers in your motors.
When you apply load to a DC motor it generates heat - the more load the more heat. The worst case for this is when the motor is stalled (throttle is applied to the motor, but it does not have enough torque to rotate), but even light use will cause the motor to heat up. If unprotected, a motor can overheat and die. VEX 393 motors include a breaker that will trip to protect the motor from overheating. If you trip the breaker, it will reset a few seconds after throttle is no longer applied. The breaker trips when it becomes hot, so if it trips once it will trip very easily until it has time to sit and cool. The motor should return to full performance after the breaker has had time to cool (15-20mins).
Drive motors are especially susceptible to tripping if your robot is heavy or geared to drive fast. Drive motors may also be tripped if your driver gets into “pushing matches” during competition.
Let me know if you have any further questions,