Lazy RPM Gage

The RPM gage in the XJ have been dropping RPM's more and more over time. I have a Scangage which is set to read RPM among other things. At idle both the dash gage and the Scangage show the same RPM. As the RPM increase, the difference become wider and wider until the Scangage show double the RPM of the dash gage.

Did anyone have similar experiences? If so, what did you do to fix it?

I would think that the computer would be reading the same input as the gauge cluster, so both should be reading the same value, which would make it a problem with the gauge cluster. I don't know if that's true however, maybe they have different sources. Do you have an FSM?