The issue that you are having might be caused by the Audio/Video configuration of the Slingbox. Make sure that you select the correct amount of zeros on the configuration and try changing the different speeds showing on the setup.
I installed a 500 today with a Motorola cable box ( I can get the exact model if needed.) I am having the exact same problem the OP described. I don't recall seeing anything regarding this on setup, did I miss it? How do I fix this? Can I change the settings or do I need to delete it and reinstall?