My problem may be ambient temperature related rather than anything to do with the A/C. Just coincidence that I am running the A/C when it is hot and the Tach is misbehaving.

First thing in the morning (with no A/C on), the tach performs normally even after a 25 mile run it is still showing the correct rpm. After the car has been sitting in the sun (70-80 degF the last few days) the tach will misread from start-up. At idle it shows the normal 500-600rpm but is way off as road speed increases in every gear. Switching the A/c on or off does not change the tach reading.

All other instruments are normal so I don't think it is a grounding issue since I would expect them all to be affected.