FWIIW, the Mustang's power windows are protected by an automatic reset circuit breaker in the engine bay and in the window motor itself. This explains why it doesn't work and then does.
If the CB is tripping, likely it's because the motor is drawing too much power. The most likely reason is worn bearings or the window is binding in the track.
Suggest starting by doing a functional test of the window motor directly from a power source. If the window works fine, this points to a wiring problem. Otherwise it's time for a new motor.