My daughter has a 05 gt mustang with 5 speed. 4.6lt. She was sitting in the car and cranked it up and held it at 1500 rpms and noticed the rpms would go up to 3000 and then back down to less than 1000. It would keep jumping from 3000 to 1000. What would be causing that? It gets terrible gas mileage btw and sometimes tries to bog down. Any help is appreciated.