Intermittent problems are hard to fing because you need to catch them in the act,
and they never act up in a good place. So,the next time this happens, if someone
is with you check for spark, remove a spark plug wire insert a screwdriver into the
boot and then hold the metal part of the screwdriver 1/2 in. away from a metal part
of the engine block and have someone crank the engine while you look for spark.
Next check for fuel by removing the plastic cap from the injection test port then
depress the schraeder valve and have someone turn the ignition switch on, fuel
should spray out. Don't reverse the order of these tests because of fire hazard.
Repost Here when you know where the problem is, I hope this helps you, take
care.