water stops for no reason.after about 1min water runs o.k. It doesn't happen every time. water pump is about 5yr 0ld. well is 550ft.plumbre replaced relay box. well pressure is about 35p.s.i.holding tank holds about 20 gal. Plumber says it mat be the pump. pump kicks in when tank is empty. can you give me some info before I replace the pump.