98 S-10

My brother has a 98 s-10 2wd 5spd 4 cyl. Pickup. He called me the other day and said his truck was not running right. He said the truck is idling fine, but when he puts it in gear and tries to accelerate, it acts like it miss fires. I automatically thought it was a fuel problem, but then he said that when he gets it up to speed (55 or so) it runs fine. I figured we would just change the spark plugs, and add some octane booster to the fuel. We are going to do this in a couple days, but before we do, is there anything I am missing? Could this be the fuel pump? Or a sensor? Any insite would be appreciated.