Missfire with a/c on

I've never seen a CEL on the 99-04 GT come on and then go off...especially without throwing a code. From what I know, when the CEL comes on it stays on until the DTC is erased. At that point the ECU does not return to readiness mode until you've driven 500 miles. However if the problem is still present, then the CEL will return when you turn off the engine and re-start it. But again, I've never seen them come and go...at least not on the OBD-2 cars. So are you absolutely sure it's the CEL coming on? Maybe it's something else.

And are you sure it's misfiring, or is it just not accelerating? 5th gear is your OD gear so you're not gonna have a lot of acceleration at 60-70 mph. Try testing it in 4th. If there is still a problem then maybe it's the compressor. Maybe the pulley is seizing a bit when the clutch on the A/C compressor is engaged. Are you hearing any weird noises or smelling any rubber? Are there any pieces from the belt in the engine compartment? Is the A/C actually working when you turn it on? How old is the accessory belt?

Well I'm sorry but I can't help ya...this is interesting so I'll do some research and see if I can figure anything out. I'll keep an eye out for ya bro...