Do you mean it moves with the key merely turned to on and off positions, or do you mean it moves when the engine is cranking/running and then again when the engine is shut off?
Intermittents like this can be hard to troubleshoot. Offhand, I suppose you could have either a harness wiring issue (engine vibrations causing intermittent connect/disconnect in a broken wire), if it moves when the engine is running or stopping, or perhaps the mechanic you had looking at it messed with the instrument cluster for some reason and caused a flaky connection at the tach terminal -- if memory serves, the tach is a plug-in module.
Try the easiest stuff first, of course. But if all else fails, you might remove the instrument cluster clear lens and tap around on the tach with a finger and see if it springs to life with the engine running. Maybe remove the tach, clean the contacts, then reinstall.
Could have other electrical issues. A battery about to go bad will cause weird issues. An alternator on its last legs can cause weird issues. Bad grounding. The list goes on...
You could take it back to the garage and tell them, "Hey, this tach was functional before you guys worked on my car, and now it's not. You have to fix it." See how well that goes. :wink2:
