Im having issues in my 91 mustang ever since i dropped my car off to have valves worked on it may be an awkward coincidence but running straight x pipe car runs flawless no surging or anything i put my mac pro dumps on and it immediately hunts at idles from 600-1200 rpms before slowly creeping down and bogging out wondering if a clogged up muffler would do this? Not letting enough air out and all