I had a similar situation this year. What happened was that the pressure tank had a leaking diaphragm. With water running and while I watched the pressure gauge, the needle would drop a little, then drop real fast to almost 0, then the pump would come on and the needle would jump up quickly to the cutoff point (60 pounds of pressure), then the pump would shut off. During the short time the pump was on it probably only pumped 3-5 gallons. Luckily my tank was still under warranty so I only had to pay for the installation. Everything's back to normal now.
Good Luck /forums/images/graemlins/smile.gif