[HO455]

To troubleshoot this problem I would let the truck get cold and then put a portable heater in the cab and point the discharge under the dash and get things hot under there and see if the problem disappears. Then you know the basic location of the issue. If it is the cluster and a ground wire fixes the problem most likely the screws that hold the cluster together have gotten loose and when the parts get warm they move slightly and the ground is restored. This is a milliamp circuit so it doesn't take much resistance to alter readout. If the heater or ground wire trick doesn't help then I would disconnect the factory tach lead at the coil and the guage and run a separate test wire through a window and see if it makes the tach work correctly. Another easy check would be to swap the HEI cap and coil with another one. Somewhere you have a spot that is not conducting until it warm. The gap is closed by the themal expansion of the components.