Make sure you have a good ground from engine to battery, probably to the frame, but make sure the ground strap/wire is of sufficient guage and the contact point is clean. Also, make sure your guage has a good ground. You can check this by running a simple test. Make a temporary ground for your guage out of 12 or 14 guage wire and run it a known good ground. Run your engine and see if you notice any difference. I experienced the same symptoms as you and the cure was in the grounds.
Jim
