I had the same problem. I tried everything. Turned out to be the cluster itself. The first things to check are:
1) Look at the distributor cap. There should be two wires hanging off of it, a white one and a brown one. Check for corrosion on them. Make sure they are plugged in correctly.
2) Check the tach filter. This is back behind the engine bolted to the driver's side head. It will be a small cylinder about 3/4" around and 1.5" long. It should have a wire going in and a wire going out. The wire going in should come from the distributor, the wire going out should go into the harness and eventually to the tach. You can cut the wire going into the tach and the wire going out of the tach and put them together. Your tach may work then. You could also replace the tach filter itself (costs around $20). This may cure your tach problem.
3) Last check the cluster. Take it off and check the connections. Sometimes they corrode. I had to replace my whole cluster. I got lucky and found one on ebay for $40. I plugged it in and my tach has worked great ever since then.
Good luck.
