Help with tach on a stock 78 280Z Please

[RPM Resto & Custom]

Hey guys I'm fiddling with my buddy's 78 280Z and having a tach issue. His original tach seems to work fine until around 2,500 RPM's and then gets stuck. Sometimes it won't come back down for a while. I assumed it was a mechanical issue inside the tach but I'm not sure. I borrowed another used tach from a local guy....he said it supposedly worked when it was pulled a while back. When I plug that into place it actually works worse than the original. It takes several seconds for it to come up to the correct RPM reading and also won't seem to go above roughly 2,500 RPM's.

I cleaned the wiring connections at the coil and the coil was a stock replacement that was just put in recently. I also pulled the resistor out from the pass kick panel and it check out at around 2.18K Ohms....I believe it should be 2.2K so I'm close enough.

Is it possible I have 2 bad tachs? That's kind of what I'm leaning towards....one bad electrically and one bad mechanically maybe? Is there anything else in the system I should be checking?