As mentioned, you pump is too "hot" for your O/F to keep up. However, there are a few things you can do to remedy this without buying a new pump.
Is your return split, or is it a single-ended return? If you add a "Y" and split the return, or even better, split your return and run it thru a SCWD to alternate the flow.
Since you're using flex tube (I plumb all of our setups this way), you can add a bleed valve back to the sump from your return like this:
This configuration allows you to bleed any excess flow back to your sump. You can easily tune your pump's return rate to your O/F rate this way, and if you ever want to run a fuge or other equipment, this may be enuff flow to do so, depending on what you bleed off. This configuration doesn't restrict your pump, so no worries there (even tho centrifugal pumps can be restricted, I don't like to do it).
Just some options...HTH