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After it it warmed up my van hesitates on accleration. When it is first started it runs ok

My 1987 Chev C20 van with the 5 liter engine runs ok when first started but after it has ran for a few minutes it hesitates when accelerating. When I press on the gas it acts like it is going to stall and when I give the gas another push it surges and takes off. I have replaced the fuel filter which made no noticeable difference and added some seafoam to my gas which did make it run a little better but did not stop the hesitation problem. This hesitation is fairly significant and to the point where I am afraid to drive the van very far from home because I am afraid it will stall. One mechanic says it is the fuel injection and wants me to have him replace it but I am not sure if he knows for sure or just wants the job for the money. It seems to me that if it was the fuel injectors then it would run bad when it was first started. So who can tell me what the problem is or how to figure it out?