Cylinder 6 misfire secondary ionization problem?!

I've got a 2007 dodge nitro with 3.7l engine. At very beginning check engine light came on with injector 6 misfire, so I replaced injector and light went away. Couple days later I tuned up the engine with all new coils and plugs. Immediately after the check engine light came back on and code for cylinder 6 misfire secondary ionization. Had coils and plug checked and all were firing properly. Couple days later my car overheated and I thought it was radiator but ended up being water pump, so I replaced both, then A mechanic said it could possibly be clogged catalytic converters so I had them cut off and wielded a pipe in place. When I cut off cats the engine quit studdering all except when you hit 4th gear at 45-50 mph the engine will studder and cause misfire. I'm at the point where all my mechanics are scratching their heads. Can anyone help with this??? Transmission has been checked as well. The mechanic said transmission is fine. Also compression is fine in cylinders, as well as fuel pressure. It's a puzzling issue!! Plz help