I had this very same problem when I first set up my RS 135. Found out that there was too much "foaming" agents in my water. Once I let the Novaqua and Amquel work itself out of the water, all was good.
Also, you need to check to see how much of the skimmer is actually out of the water in the sump.
Make sure you have the foam sleeve pulled up high on the output.
Hope that helps..........