Try this - its free and even though it is primarily for 5.0 fox bodied cars, it may help you fix your problem.
The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C. Make sure that the ground places are clean and shiny bare metal: no paint, no corrosion.