I was driving down the highway at app. 3200 rpm's and suddenly my Tach drops to 2500 rpm's. The engine speed remains the same speed. Is this a Tach or coil problem. Doesn't the Tach receive a pulse from the negative terminal on the coil. Both the coil and the Tach. are originial to the car. The drop on rpm's last about 15 seconds then jumps back up. At first I thought it was a loose wire at the coil, but I don't see a problem there. This problem doesn't happen all the time.

If the engine is continuing to run correctly it is the tachometer or its associated wiring. I would start by looking for bad connections in the wiring for the tachometer.

260Z tachs were, for some reason, not very durable. I have a feeling your tach is slowly dying. Mine worked intermittently and then died so I replaced it with a 280Z tach and never looked back. Switch the faceplates out if you want to retain the 260Z plate and you're good to go.