tach spasms

Hi folks, I hope someone has an easy fix for me. I recently bought my neighbors '91 sentra. I thought the tach not working at all. However, periodically, the needle springs to life and gives me, what appears to be a correct reading. Other times, it spazzes out and flips back and forth between 0 and 9000. Loose connection? Bad sensor? Please advise. Thanks :feedback: