99 Mustang GT won't stay running...

My car is doing the same thing. If I am going about 50 and put it in neutral to slow down it will drop RPMs to around 500 then go down to around 300 and sometimes die. I can not get it to reproduce the idle surge unless I am driving so I can not pull off the TPS to see if that is whats wrong. So far I took off my MAC and went to stock with a K&N drop in. Ive checked for leaks over and over. Someone told me my programmer or my throttle body could be messing with my air/fuel mixture really bad, but all symptoms point to a TPS, IAC, or a EGR. Any ideas?

Just wanted to say thanks for the help...my 99GT would not stay running unless I kept my foot on the gas. It only did this every once in awhile. Now I know what may be causing the situation. I went out to the garage and used some carb and choke cleaner on the IAC...so far runs fine.