91 speedo drivin me nuts

did a sye.replaced the vss and gear.(original was shot)got the speedo to wrk.worked for about a week and then over 35 started jumpin all over the place.played with it still couldnt get it to work properly.returned the vss about 2 weeks ago for another and worked fine for a week.now its doin the same thing!i could see if it was skipping a tooth or something that the speed would drop but what could possibly make it jump way up to 85 and back?

Are you clocking the gear housing correctly when reinstalling it? The holddown needs to click into the right part of the housing to keep the gears meshed properly.