my 95 ranger 2.3

i have this problem with my truck not wanting to start at times. it turns over but won't start.after i let it sit for a while it'll start or sometimes i'll crank it again and it'll start.replaced the fuel relay switch and it didn't help..it did it to me today at the vet's with my cat.the engine light don't come on so i'm really stumped.any idea's on this?oh ya, it's not the gas cap.i was told it could be bad and would make it do that.not it either.
thanks, mary