some times engine will not run. Battery ok starter ok After an hour it run

Nothing wrong with starter, engine will not run, after waiting for 1/2 hour I turn the ignition key to start; and wala! Engine run and is drive able again.( without do any ting, just wait) Maybe the gas line relay is the problem. Or stop working wants a whale. Engine need a spark from the spark plug and gas! how to check this? It can be done only when the engine will not run?