Do you have an inlet filter (screen) on the pump? If not, you may have a piece of plastic from the water tank or some sawdust from construction that migrated to the pump head. You can remove the pump head, disassemble it "CAREFULLY" and clean it. FloJet sells a "clip on" inlet screen that will eliminate any further problems. On reassembly, make sure you aren't "sucking air" on the inlet side. A little Vaseline or silicone grease on the hose fitting orings will help tremendously.
Here is one source for the strainer:
http://www.amazon.com/Flojet-0174030...+pump+strainer
Good Luck