1988 Ford E-Series Van Engine loses power (hesitates)

Engine loses power (hesitates) during take off/ Acceration/ and while crusing. It's not a constant loss it's on and off sometimes it will do it and some times it won't. While it was hesitating I floored it and it backfired. I've put new plugs and wires in it. It ran fine for alittle while then it got bad again. Usually after it's been running for a long time way after it's at operating temp it runs ok and doesn't really hesitate anymore while accerateing or cursing but sometimes on take off it will hesitate for a second. I can't figure it out I'm thinking fuel pump or electrical but don't know how to narrow it down. Thanks for your time