I have a Polaris 380 that is about 4 years old. I have bought a new pump, opened up the 380 and checked everything inside. All is good as far as I can see. The unit when it is in the pool is real slow. I do notice when I take it out of the water it starts to run a lot faster but the minute I put it back in the pool, everything slows down. What could be causing this unit to run so slow?