If you can remove the cluster, look in the Yellow pages for a Speedometer repair shop. Sounds like you have an electronic control mechanism burned out (possibly) and the tach probably won't hold a set even if you can adjust it. If there is no one in your area, Palo Alto Speedometer in California does them, and they'll give you a price first. Bill's Speedo here in St. Louis, MO does them also--depends on your location.
If you want to try it first, take the cluster out of the dash, and look for a rheostat (small screw, usually in a little metal box) to adjust the swing. BE CAREFUL! Use a tiny flat blade screwdriver, start the engine with the tach plugged-in, and SLOWLY turn the screw while watching the tach dial. If there IS an adjustment, and you DO get it adjusted, and it doesn't hold, it's what I said previously and you're gonna have to send it to a pro.
