tach works, then stops on my 96 300zx tt

[ernie]

I have a 2012 370z roadster, all the bells and whistles, nav, sport pkg, even have a spoiler, great car, fun to drive. I also have a 96 300zx tt that I am working on, it was totaled, has a rebuilt title, runs and looks great. the interior is all new, I installed chrome bezels around my gauges, what a bitch of a job for a 66 year old with artheritis in his hands. To make a long story short, I tested all the gauges before I buttoned it all back up, everything worked fine. Two days later, my tach stopped working, some times it would shoot up to 5500 rpm, then just drop back down. yesterday it worked great, no issues, today, nothing????I have very little knowledge about cars, I just love them. Any help would be greatly appreciated.