It could be a switch or a motor; both commonly fail and give the symptoms you describe. If the window is moving very slowly, it would be more likely to be a worn motor that is overheating and tripping the circuit breaker. If it seems to move normally but works intermittently, then it could be either a motor or a switch: the only way to know for sure would be to test for power and ground coming out of the switch to the motor when the problem is occurring.
A couple quick tests that might be informative:
(1) If whacking the door sharply with the palm of your hand causes it to start working again, this would tend to indicate a failing motor with an intermittent internal connection
(2) If wiggling the switch button around makes the motor move at all, even intermittently, this would indicate a worn switch.
If you still have no idea, and have no means of testing it, try installing a switch; they are relatively inexpensive and easy to replace... If it turns out to be the motor, this is probably a job you want to take in for a technician to handle anyway; changing window motors in these is not fun at all, as everything is riveted together inside the door making service difficult to say the least.
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