[Al Skierkiewicz]

The reason your robot is not moving very fast is because the motor is running near stall (due to the load you are giving it)and therefore at a very high average current draw. Since the 30 amp circuit breakers are "thermal" devices, they will trip with high average currents and then will continue to trip more often after the first time due to the high internal heating. A quick fix would be to slow down the drive in software but the gear ratio change is what you really want to do. Even with a good gear selection some motors will heat up. Take the Fisher Price for example, it has a really inefficient fan inside to circulate air. Block the holes with a mounting bracket and you are just complicating the problem. To add to all that, the fan is just "friction fit" on the shaft inside the motor and when the motor temp reaches the melting point of plastic, the fan stops.
Although there is no track record on the "Chalupa"
motor, I am going to guess it will have a serious heat issue. It is designed for intermittant duty in an all weather environment so there is no air movement through the motor. The best method on this motor is heatsink in the mounting design.
Please make sure (with accurate measuring on a regulation playing surface) that your motors are not running near stall speeds and adjust gearing if they are. This will insure long life on motors and NO tripped circuit breakers.
Good luck all,
Al