1996 Ford Windstar van wont start

OK I have a 96 ford wind star that won't start she cranks but no start. I didn't check for spark or fuel but what makes me crazy is wen turn ignition switch on if the check engine light appears on the dash it will start but if I doesn't it will crank but no start. What can it be? Already replace ignition switch crank sensor and used ecm that I give the vin to the junkyard and it came programed. Wen replace the ecm it starts and drive fine for a couple days one day was driving and van shuts off on me. I cycle the ignition and starts again. Next day was at gas station filing up and till now won't start uneless if the engine light comes on it will start. Thanks again pedro.