It is drawing way too much current and the circuit breaker is opening. Each time the breaker trips it is de-rating itself so it trips on even less current the next time, repeating the process until is then worthless.
1) Remove the prop and check for fish line wrapped around the shaft.
2) with the prop installed, manually turn the motor to check for signs of tight bearings. The prop with have a slight jerkiness but that's normal. The prop however, should be hard to turn. If so, it needs to be opened up for bearing or other internal repairs.
3) If the motor appears ok, check the connections at the battery, at the breaker, and at the connector where the troller plugs in as well as the wires in the plug and receptacle. Corrosion at any of those points raises resistance and resistance is heat..