Some time back I spent an entire saturday trying to fix the exact problem you are having. Above 1200 RPM my tach works great... at idle it goes to zero. I pulled the dash apart and cleaned all the contacts with a small copper wire brush and contact cleaner. Pulled all the grounds off I could find and wire brushed the wire eyes and the chassis, put copper anti corrosion compound on all the engine compartment grounds, contact cleaned the connector for the tach sensor, etc....
Still does the exact, same, thing....
Although I did completely clean the inside of the gauge cluster and replace a bunch of failed lamps, so it wasn't a total loss.....