Please help. I have been having some periodic loss of pressure while the pump is filtering. When the jandy valve is on drain it works fine. If I start to move it toward skimmer air starts pulling through and the skimmer is not pulling water into the motor completely. I don't lose all pressure but it is very low and the motor basket area has lots of bubbles. I use skimmer basket socks to keep large items, bus, etc from getting into the filter. The motor basket is clean and so is the filter. No leaking anywhere. Any suggestions?