I was going to shim my fpr on my truck and thought I would get a baseline reading on my fuel pressure before I done anything. I hooked up my guage and started the truck. When I looked at the gauge to get a reading, the needle was jumping all over the place. It was going from around 45 to about 60 in a steady ryhthem. What is going on here? Any help would be greatly appreciated. Thanks y'all!!