Airguide speedometers had "balance tubes" mounted at the rear of the boat on each side of the fuel tank. They are a brass tube that the speedo hose from the pickup feeds into and then on to the speedo itself. They can become plugged with silt, sand, etc. and cause readings like you have described. You can try flushing them out with water, I would try taking them out to do this, and blow the lines clear while your there. DO NOT OVERPRESSURIZE, you can blow the line, caps off the balance tube, or the speedometer itself.
