My 96 did it when I first bought it, found out it had been doing it since it was new...he was just too damn lazy to check it out. I also had an erratic speedometer...actually reminded me of the old days when you would have to lube the speedo cable...it would jump around sometimes, or just plain not work. Found out from a GM tech buddy of mine that it wasn't too common of a problem, but it did happen on some of the trucks. Turns out the gauge went bad, no ground or fuel pump problem. I replaced my cluster and it has worked fine ever since.
Not saying that would solve your problem, but that's exactly what mine was doing, and it solved it.