Clone my skimmer starting doing this last week after making some aquarium changes and doing about a 25% water change. Its still acting the way yours is a week later. The only thing I could do to help was put some filter material on the pump intake to slow it down and it works, but when I take it off it still fills the cup in seconds (actually pops the top off). So Im leaving the filter material on the intake untill it breaks itself back in, however long it takes.