I have had similar problems twice with my skimmer (ancient EuroReef) and the solution (twice now) has been a new impeller (indecently expensive but cheaper than a new pump).
It seems that the magnet swells over time, even with no signs of rust, and starts binding on the inside of the pump - sometimes you can see light scoring on the magnet.
I've had the same issues with the Jebao wave makers too, but haven't tried new impellers in them since I didn't make the connection until about 5 minutes ago - duh!
One day I'm going to make a tiny lathe to turn down and then epoxy coat the magnets again - has to be worth it given the last impeller for the skimmer came in around $90 by the time I'd had it shipped and everything..