If the engine runs well once it starts & doesn't smoke I'd rule out rings. I find temperature related issues are usually electrically based. I'd check the timing while cranking the engine cold, then again once the engine's hot. If that checks out then I'd check the fuel shut off solnoid- sometimes they stick when hot. Supply external power to the propane shut off solnoid & run engine until hot- turn off propane bottle til engine stalls (leave propane solnoid energized)- wait a minute & try to restart- if engine starts good- shut off solnoid is faulty- hope it helps