Christopher Bonnie

Hello, first post of many, I have a 94 cherokee sport automatic and have had this issue since I bought it a month ago. If I put my foot on the gas gently or if I floor it, the engine runs fine but holding the throttle around the middle (like trying gas to maintain 55mph on a hill) the engine will buck, it jumps back and forth around 2-3000 rpm. It'll go away after about 10-15 seconds or if I let off the gas abit but im concerned. I know I have an exhaust leak at the headers because I did a freeze plug job and the bolt stud at the very back of the exhaust manifold pulled out and when I seafoamed it a nice cloud of smoke came from under the hood but I don't know if that'll cause this, any advice?