My in-dash tachometer works most of the time but occasionally just drops to zero and rests. If I shut down the car and restart it works again. What is the fix?

Check the ground wire running off the tach (wire from back of tach to ground), usually a poor ground is the main culprit with most electrical gremlin's. Next I would recheck the tach lead (wire from back of tach to negative post on coil), and finally your ignition lead (wire from back of tach to ignition terminal at fuse box). There is no more. After that you may need to have the tach bench tested.
Good luck!!

I still have problems with the in-dash tachometer failing. If I tap the lens cover with my finger tips it often comes back to life for a little while. All the grounds are fastened securely and the tach is connected in the back with a plug-in wiring harness, which itself is new. It is hard to see behind the tach, but is there a terminal that I could attach another ground wire to?
Any suggestions beyond removing it and bench testing.