Subaru's intermittent starting

My 1996 Subaru Outback 200,000 miles occasionally will not restart when it is at operating temperature and has been turned off for 20 minutes to 1 1/2 hours. It has so far always eventually started after trying it every 5 minutes for up to an hour. It starts and runs normally ie no sputter or rough idling once it decides to. It will always turn over but will not start. This has never happened when the engine was cold.