I had something similar happen to me last fall. We had a lot of leaves drop in just a few days, and I wasn't as diligent as I should have been at checking the skimmer basket. It had filled with leaves and been sucked down to the bottom of the skimmer. In doing that, it ripped the part of the flange off the basket and punctured the bottom with a couple of fittings that are in the bottom of the skimmer. Could the skimmer sock be getting clogged and causing the pump to pull really hard on the basket?