AC/Cooling System/ Spark Plugs

I changed the air filter and spark plugs this weekend, and now my Service Engine Soon light is on. I have a 99 Ford Escort 2.0L.
The new plugs were Autolite 5144's with a .052Gap
Air filter was the Fram air filter from Walmart.

I'm a DIY mechanic with a little experience, and I would think I'd be able to tell if one of the cylinder's is misfiring, or not firing at all. Is there an easy way to check. I'm going to replace the wires this afternoon.

My AC has been acting up since Friday as well. When I'm at a light/ Not moving, the air compressor cycles on and off a lot more than normal and just blows warm air. When I'm moving at a good rate, without many stops, the air works fine. I added Freon, and there is plenty of charge.

Also, I added Coolant yesterday as my reservoir was very low.
Today when I was about 2 miles from work, I was almost in the red on my temperature guage, which was very abnormal, I turned the heat up, and the temp went back down.
Any ideas as to the "Check Engine Light", The AC not working, or my Overheating?

Got the Computer checked at Advanced Auto Parts, Came back with a P0443 Code
Evap Emission
Control System
Purge Valve C Fault

So far I've found in the forums that I need to check the IAT Sensor,
and possibly the DPFE

Since I just changed the air filter, and it hadn't been changed in a long time, perhaps it's the IAT.
Any idea where it's located?

Check to make sure that the gas cap is tight. Also make sure all the vacuum hoses are properly connected. As for the AC problem I think you have a fan issue. Check the fan to make sure it is running. This would account for the air not working while sitting still and the overheating problem.