I’ve had my 98 GT 5 Speed for about 3 months now, when I got the car it had a bad exhaust leak at the headers and also had the CEL on due to the TPS and ECT Sensor giving high voltage. Turned out that the sensors shared a harness and were not grounded. Something must have happened to the wire but a new ground was made and now the sensors work as they should and no more CEL, exhaust leak has also been fixed with new BBK shorty headers. Now the only thing left is the idle fluctuation. When the car is idling fine at around 700ish and I press the gas even a little the idle will drop to around 300-500 and then pop back up to 900 and then idle perfectly fine again at 700, if I just tap the gas again it does it all over again. It only drops after I tap the gas and the idle comes down very low. There are no accessories on when this happens. If it’s not doing this hiccup then it’s just going back and forth constantly between 500-900 rpm, back and forth until I shut the car off and turn it back on, sometimes it stops and idles at 700 again, and sometimes it doesn’t and continues the fluctuations. I’m 99% there is no vacuum leak, we sprayed around with carb cleaner and nothing, I disconnected the iac and the engine dies right away as it should. The TPS is brand new, the IAC is brand new, the MAF is brand new, the PCV valve is brand new. Car has bbk shorty headers with catless x pipe and possibly an aftermarket cat back, not sure. Has a COI, 73 mm BBK throttle body and msd coil packs with ford racing wires. This car has had this issue since I’ve gotten it and I figured it would be gone once the exhaust leak and tps was fixed but I’ve got no clue at this point. Air compressor is also new from a previous owner but the A/C does not work. Don’t know if that’s important or not but yea. Does anyone please have any ideas lol