Well....it happened again, starter didn't have the oomph to turn the engine when hot. Pulled the plugs and it turned over, cleared any gasses out of the cylinders. Reinstalled the plugs, checked the battery (12.4 volts). Luckily I got a push from some Samaritans and went home. Last night I pulled the starter off the other (retired) motor, made sure to clean the grounding points with fine sandpaper, wiggled the contact posts, and I didn't push the starter all the way into the engine case when re-installing. Too late for a proper test of hot start but the engine did seem to be turning over much faster than before. Too bad I touched on so many variables because if I did fix it I won't know what the problem was for sure.