My guage started doing the same thing a week ago. I changed the fuel filter thinking it needed changing and my guage was still all over the place. Called the local dealership and told them I suspected the lift pump. Service writer asked if I was having any problem starting the truck. I told him "No" but that I knew about the potential problems associated with a bad lift pump. Took the truck in this past Monday after work and was called yesterday and told that my lift pump had been replaced and the pressure is fine now. I'll be picking up my truck this afternoon. My truck is a 2002 with less than 9k miles on it and I've already had the transfer case seal replaced and the lift pump under warrently. I will admit, this dealer's service dept has been outstanding, I bought my truck in Idaho and this dealer is in So Cal. Both times they have had my truck in and fixed in less than 24 hrs. I might just buy my wife a car/truck from them next time. As far as I'm concerned, my fp guage has paid for itself. Without it I might have ended up stranded. My problem has convinced my father, who has an '01 to put one in his truck. Good luck.
Bocifus