new jeep new problems

Recently bought me a 92 YJ stock accept for the smittybuilt bumpers and steps(which have been removed) Anyway Im having an odd issue and im looking for ideas on what it may be. first its a 2.5 5speed, this morning I drove it to work ran great. This afternoon it would buck and back fire at around 2500 rpm. Whats weird is I only noticed it in first and second gear. I noticed it was running cool right around 170 so I came home and changed out the Tstat tested it and it was back at normal temp, thinking the old tstat was sticking open causing it to run rich. Still have the same problem. I also cleaned the TB which was pretty dirty. The day I brought it home I changed the plugs/cap/rotor/wires. I looked for vacuum leaks but couldnt find anything. What am I missing?