[RGordon]

Sounds to me like you have a air leak in the suction of the pump, either the valve for the external line for antifreeze insertion or the clear strainer bulb on the suction side of the pump. My strainer bulb cracked this winter with antifreeze in it (another lesson learned for me to drain after winterizing system). I would find the filler line for antifreeze and align valve to line instead of tank and like someone else mentioned, place a jug of water and insert hose into it and see if this makes a difference. I would bet you have an air leak in the suction strainer area causing pump to cavitate and fluctuating water pressure.