1991 era 6.5 Kohler genset. Cranks easily, runs very smoothly for about 40 minutes under load, then dies. Good waterflow. Will crank immediately after it dies and runs smoothly for about 10-12 minutes, then dies. Does this regardless of load. I would think that a temp issue would prevent it from re-cranking right away as it would still be hot. oil level fine. ideas? thanks in advance.