this happened to me and what the propblem was is that there is a small set of gears that are behinde the instrament panale. i would pull the instrament culster out and check there first.they are all plastic and the main gear that moves the trip odmeater and the regular odometer has broken teeth and needs to be replaced. out here in califronia the part from the ford dealership is $400 bucks.(if you buy the part from ford you have to buy the tach and the speed odmeter you cant just buy the gear.) dont waste you money go to a junk yard and pull the peice from another mustang. hope that this helps.
