My car has been seeming to be running fine lately but i was late for work one morning and decided to go a lil over the speed limit once i hit 80 my rpms just stop at 4000 and the car loses any chance of going faster. the check engine light usually comes on than off but i got it to stay on one day took it to autozone they scanned it and said it was a cylinder 4 misfire so i went home replaced the spark plug on 4 and than went out to test it and yet again same problem any ideas?