I'm driving an 02 caravan SE 4 cyl. Last March it started to run bad. Stutter and lunge. Brought it in and it won't throw a code. All it says is random misfire on all 4 cylinders with no pattern and no codes. After it warmed up in summer it ran like a top and now with colder air outside again same problem. Some days it runs great and others very sluggish. Anyone have any ideas? I've already replaced the fuel filter, plugs , ignition wires, complete tranny flush and cleaned out the fuel system all per my mechanic and nothing helps. Close to a thousand dollars spent and still no fix. Help!

 

I would suspect a vacuum leak and probably at the intake plenum seal.