Engine My '99 GT is surging @ idle...HELP

[99 FLA PONY GT]

I recently made the road trip from Brooklyn, NY to Ocala Florida for the 19th Annual Silver Springs Ford & Mustang Roundup sponsored by NPD. The car ran great until I hit Georgia and filled up. I have a Steeda chip and tune from 1999 and was told by Steeda when I lived in S. Florida to always run Premium gas due to the chip/tune. I always have up until this long roadtrip so a couple of tankfulls I ran midgrade and the car ran the same...no problems. I then got gas in Georgia and as soon as I left the station I could tell or thought I got a tank with bad gas. I had run Chevron Techron before I left NY and then when I experienced a rough idle, I put another bottle of Techron in with a tankful of Premium gas. Nothing, the car would still surge at idle from 400 rpms to 800 rpms and rock as if I was playing with the brakes rocking out to a cool song. I also bought a bottle of dry gas to remove any water from my tank and ran 5 complete fill-ups of Premium all the way back to NY and the car is still surging so maybe it's not a bad tank of gas afterall.
I don't have any check engine lights on indicating a bad sensor.
Any thoughts??????????
Thanks in advance.

 

[PBullitt3255]

400 rpms isn't that very low?.. Doesn't mustang req premium? I have a 01 Bullitt and it says it right on the gas cap...I live in S flo and heard the opposite from my gpa. that you can run lower grade here bcuz it is so flat its easier on the engine, and always use premium upstate because there are so many hills/inclines its harder on the engine, not sure if this is really true or not. But of course down here you need to keep your coolant system in check...

For the rough idle not rly sure why it would do this and not throw codes, if you don't get any better advice to fix it try testing your battery and alternator..maybe the alternator is going bad, never the stang but I had two cars that one would steady rev at about 1500rpms and one would be a rough idle(honda accord) jumping how you explained but probably 700-1500 rpms instead of 400-800.....

Good Luck!

 

[trinity_gt]

It could just be a coincidence that your IAC valve decided to screw up during your trip.

 

[cj04gt]

could be vacuum leak somewhere get a can of 3m brake clean and lightly sprat around intake if its a vacuum leak it will raise and smooth out the rpm

 

[nine2oneGT]

I had this problem before, tried cleaning the IAC with some MAF cleaner, didn't fix it. I ended up replacing the IAC, problem solved.