Wire connections. Clean 'em. I'm leaning towards electrical problem because of the intermittent nature. As things heat up they expand. Electrical components can act up when heated.
Check your plug wires and boots. A great product is DeOxit. A small can is expensive but this stuff REALLY cleans electrical connections. Don't try to save and get something at the auto store,it ain't the same.
BTW how's the charging system?