Stopping wipers

The wipers on my '65 Barracuda will stop after running for several minutes. I know the switch has a circuit breaker. I had the s/w out of the dash and in my hand with wiring still connected and ran the wipers. I don't feel a click in the s/w when the motor stops but touching the motor when it stops I feel a click. From what I can find there is no circuit breaker in the motor. At first I thought the problem might be stiff wiper linkage. I checked and lubed the linkage and it seams OK. I removed the wiper arms and ran the wipers but the motor still stopped. The motor is warm but doesn't get hot. Does anyone have any ideas as to the cause of my problem?