Pumping from a stream for irrigation and am having trouble with pump output. It's been in use several summers and worked well until recently. Checked the input screen, filter, etc. Finally decided to see what the volume of output was. At about 40 PSI the output is only about 10 gal/min and normally I have about 15 gal/min at 60 psi. At cutoff I get about 80 PSI which seems about normal. Don't have any leaks that I can find. The water does have a small amount of sand in it. I have a fine screen on the pickup but some makes it through and gets filtered after the pump. I don't see any leaks from the seal although there might be a very slight one. Impeller and jet are plastic. Has the sand eaten away the plastic so I need to replace them? What other checks should I be making before I take it apart?