Have a full house cistern with a submersed pump in the bottom of the 3500 gal tank. In the crawl space is a pressure tank with a rubber air bladder and a pressure switch. When I turn on the water it will run normally till (I guess) the tanks water is drawn down (about 5 gallons worth) then it slows down to about a quarter of the stream, and then in about 3 seconds it comes back up to full pressure. With a long shower this will happen several times. My guess is the air bladder has a leak. Any outside chance it could be the pressure switch, or the pump??