Possibly caused by poor connections at the battery and ground connections. Thoroughly clean up all connections involved in the starter circuit. When this happens, check the battery cables for heat....Barners; My 51-F1 EAB/ 6 volt starter spins fast and starts fast when cold, after running for 10-15 miles won't spin fast enough to start, let it set for an hour and it's fine again. What is going on here???? Jerry