Engine idle ect...help?plz

I just recently bought my 1st mustang. a 2000 GT with 59k miles. i love the car, i have always been a Chevy guy, this is my 1st ford anything. I am no mechanic by any means but dabble here and there when i can.

Anyways, heres my problem. The car won't idle, never stalls but it drops between 500-1000 rpms. I was thinking IAC? The car is also backfiring when I'm just sitting there or coasting. It also hesitates under heavy throttle. The service engine light came on, I replaced all the plugs, and it led to a coil i changed both and it went away and car felt a little better. A day later its back on still giving me issues, haven't had the code read yet, but any ideas?

Thanks

i checked out the service light today and came up with the IAC, misfire in cyl #4 and running to lean. i picked up a new IAC and going to throw it in tomorrow. the misfire.. i replaced the coil and plugs and it went away but now its back in the same cyl? maybe the wires.

injector cleaning definitely worth a shot before i start messing with the timing.