I would suspect the driver stage to be defective, probably one of the transistors especially as you mention that this gets worse with heat. Resistors are usually quite dependable, but the capacitor used for timing might have a reduced capacity due to aging. As a quick test, if you remove the blue wire from the coil/resistor, does the tacho still goes to 1000 rpm with the ignition switched on.

Todd, do you have a 4 wire or a 3 wire ?

I finally got back to the z car wiring. I finished my maxi fuse, gm alt. and zx hei ign. projects. I hooked up the tach without the resistor and compared it to an external snap on tach and dwell meter. The car runs and sounds great and both tachs read the same so I think I will leave the resistor out for now and see how it goes. Thanks everyone for your help

Bob

My tach is definitely a 3 wire. I'll have to wait until it comes back to try the wire removal test. The shop I sent the tach to wasn't able to figure out the problem.

There is a shop near me where the guy said he replaces the entire guts of the tach with modern components. However, I think he wants $190.00 for that. That's why I'd prefer to fix what I have.

If so, you can use a 260Z / 280Z tach, put the guts in the 240Z tach, you can also try to pull a wire from the single terminal on your tach (yellow wire) and run it directly to your coil.

Chris