My car is a 1999 GT coupe 4.6L V8 SOHC MOTOR. I've been having this problem since January but living in Brooklyn, NY, I only drive my car on my days off. On my roadtrip down to Silver Springs, Florida for the 19th annual Ford & Mustang roundup down in Florida, my troubles began in Georgia. I have a chip in my car that's as old as the car installed by Steeda in S.Florida. They told me from day one, drive using premium which I always have. During my trip down to Florida, I used mid grade gas & the car ran the same and good until I filled up again using midgrade gas in Georgia. As soon as I pulled out of that gas station, the car ran rough at idle when at a light. The revs would drop from 400-900 and the car would rock back & forth as if I was playing with the brakes and rocking the car to the beat of a good song. I thought at first it was bad gas but I added techron, then dry gas & now 10 tankfulls of premium gas...so I think I've ruled out the bad gas theory. Today I changed the IAC valve, platinum motorcraft spark plugs, Pcv valve, & fuel filter. Now my car is running even worse as it's now stalling out constantly when I hit the brakes at a light to stop the car. I have to wait for the light to just about to turn green then I put the car in neutral, rev the motor & then pop to car into drive as I rev the car up. What could my problem be? I've checked the intake for a leak with Water & it appeared that's not the problem. Do anyone thinks that it could be the TPS-Throttle Position Sensor? Any help much obliged.