NOS tach problems

I purchased an NOS tach from another fabo member, and I am having a problem with it. It is a Dixco column mount, 10,000 rpm. It was the one I really wanted. Here is the problem - it seems to be quite accurate at an idle, but as you rev it up, it goes much higher than it is suppose too. Idle at 950 is OK - 2500 on another tach or timing light shows 3000 on the Dixco tach. I had all three hooked up at one time and could compare them. Other than being internal - any suggestions? Thanks Darryl