This happened to me & it turned out to be a cut in the small vacuum line going to the gauge. The line is about the diameter of a small straw, grey in color, and somewhere underneath the drivers side cowel. Follow the entire line & look for a partial or complete cut. I believe it starts around the bypass valve. Mine was cut about in the middle, maybe 20 "s after it started. I think it melted from heat. I did not look into replacing it, so electrical tape has currently held it for a few months.
Hope that helps..