1998 Jeep Wrangler Jeep's Bucking on Acceleration

I just bought a used 1998 Jeep wrangler as a "fun" third car. I have had nothing but problems with it so far. After I purchased the car within a day the check engine light came on. I took it to a mechanic and he pulled the code. It was misfiring on the fifth cylinder. So we change plugs, didn't work, changed plug wires didn't work. I tried to get my money back from the seller didn't work. He di however agree to take it to a Jeep dealer. It needed a valve job. I got him to pay for it. But, he had his mechanic do the work. Now I get it back start driving it for a couple days and it feels good seems to have the power it should. But now the jeep starts bucking like crazy on acceleration usually just after shifting from first to second gear occasionally in lower rpm in third gear. It acts like it wants to stall then starts bucking. I tried using some fuel injection cleaner no help still has the same problem. I'm going to take it back to the mechanic who did the valve job but I want to know if you have any ideas or suggestions on what might be the culprit. I'm hoping he didn't screw it up worse than it started.