2003 Ford E-Series Van too much advice

Hello my van has had an intermittent miss for three months I went to firestonethey said it had an intermittent miss but didnt know the cause I talked to a friend of mine he thinks the van has so many miles on it the problom might be the timing belt or chain coil packs cost too much to go replacing all of them I think it is in the throttle body or something becaus the van sputters when I first take off. I hope there is an easy way to figure this out it is getting worse by the week please help terry