Tach flutters Sometimes, on my 87 4Runner, the tack flutters about 500 rpm between 2500 to 3500 rpm, mainly with cold engine.

I think should be around 750.

when is the last time that you did a tune up

Not sure about the tach, but the link should give you ideas to check on.
Do It Yourself Diagnosis and Repair

Chances are its not the Tach. Look over the wiring to the tach. Its it brittle about to crust off? Is it shorted to ground? Now that would pin it. Check its connections off the coil. Engine side of this item is usually where you'll find the problem. This is nothing new with a vette definately not serious. Remember the tach responds to electrical signal it gets. Before you do the surgery on the gauge itself make sure nothing is shorting this out.

Probably has bad circuit in ignition module. Remove and have checked for free at any major parts supplier. If it checks out, suspect coil.

it could be the electric choke on breather intake

you need to replace the throttle position sensor on the intake.

I had a Renault Espace that did a similar thing when it was cold, The ECU thought that the engine was -40* according to the Renault engineer. There are two sensors located together on our Espace, it ended up being the temperature sensor next to the coolant temperature guage sensor located on the thermostat housing. I hope this is of use to you. Ian

Sound like a tack or speed-o cable. you will have to buy the cable first to see how it looks; that will help you see how to replace it.

Tach is short for tachometer and is a device that tells you the speed of the engine, generally in RPM X 1000. A typical speed for a 6 cylinder engine is about 600 to 1000 for idle in Park/Neutral, and 500-800 in Drive or Reverse. This was if you have this on your dash, and can get me some numbers, I can better assist you...accordianman