speedo's and eventually usually the rest of the cluster go out on these cars all the time. Ive heard of some people finding the problem and re-soldering it but i was never able to find the problem on mine. My suggestions is to just go pull one out of a junkyard. if you go to a pick-n-pull type place they are usually about $25. Id buy the warranty on it since you may want to return it if it doesnt work. Also make sure if you have a manual to pull one out of a manual car and vis versa. Also make sure you get one out of a car that has the same engine as you and same obd...either I or II. You can also find them on ebay or vw classifieds. Another option if you dont want to fix it, which i did for a while before i found a replacement, is just to you a garmin or tom tom for your speed. Often they even have a screen tht displays your speed on the whole screen so its more visible.
Mine was intermittent for a while, and then it quit all together...speedometer-tach-oil, fuel...the whole enchilada was dead. Drove like that for about a year ... Till I made my way to the junkyard and pulled another cluster as mentioned above by another poster. Then it was all good.