i have a starting issue

2000 e-350 van I've changed,lock cylinder,ignition switch,pump relay,pressure reg.checked fuse's tested wire's.sometimes you hear the pump run and the van will start but you don't hear the pump it won't start.if you cycle the key several times the pump will run and the van will start and run just fine.usally when it's cold out it seem's to take longer  to start.