Car is hesitating.....

Hey guys, once again the stang is having problems. I fired her up this morning and all was well. I let her warm up a bit and then proceeded to get on the highway. The car had been idling just fine and there was no problems under normal acceleration. Once I was in 5th gear crusing along the highway at about 65 mph, and tried to accelerate just a bit she started hesistating. But if I backed off and just kept it constant, she would go back to normal. Just meant I couldn't accelerate very quickly if anything. Once I got off the highway about 20 miles later every 5 or so seconds, the car would dip in rpm during idle. I could aslo feel the shifter shacking as if one of the cylinders wasn't firing or something. I checked the wires and everything seemed good so im not sure what it is. Sometimes Ill fire her up and she is a little hesistant at idle, but after warming up she goes back to normal idle.I haven't had anything work done on the car recently so nothing has been fiddled with, altho it was about 20 degrees outside last night if that would effect anything.

Anyone have any ideas? Ill clean the AIC out later today.

Is there any way to tell which one? What are the prices for replacements? I have a new set of plugs I could put in. But never had to replace a coil and have 60k miles on the car.

Thanks for the reply! Im taking the car in on Tuesday for the 60k mile tune up. I may buy a new set of NGK plugs and have them put them in. I dont have any of my tools on campus.