I was having a rough, kind of rumbling, idle on my 5.2. I replaced the plugs with the correct type (previous owner had installed iridiums of the wrong heat range and type) and a new cap and rotor. Engine got much better for a while. Then the rough idle was back. So I did a search and the posts indicated a dirty throttle body and IAC valve could be the culprit. Well that was the case with a 92 Mustang 5.0 I had, so I thought I better have a look on the 5.2. Sure enough there was enough crud around the plates to cause the IAC valve to go nearly full open I guess. I also had a slight surging at cruise power around 30 mph.
I bought a Felpro TB gasket ( $2.21 my wholesale cost!) and pulled and the IAC valve. Fifteen minutes later all was well.
Thanks to this site that's about the cheapest automotive repair I've done since the 1970's!

 

I just did the same thing on my 4.7 this weekend, and now it runs like it has 30k instead of 140k.