It seemms to be a wiring or signal problem, most of all becasue you're using Coax, connect the Coax cable that you have for the Slingbox into a TV, if you get the same results is the coax cable the one with the problem, and also use a different coax cable

Viper

Actually, what you are seeing is part of the actual broadcast signal. All analog television signals broadcast a few pixels of extra information (usually useless) in either the top most row of the image or the bottom row. All consumer televisions crop off those areas. That's where the term title safe and action safe come from.

Your slingbox does not crop this however. You can confirm this by pausing a signal and looking at your computer screen and your tv. You should see more on your computer screen. Another test would be to hook up a different source. Connect a DVD player to your slingbox and see if you see the white noise still.

It seems that your cable providor might be broadcasting this extra information in too big an area. The only solution, which you're going to hate, is to degrade the signal a bit. Try hooking your coax cable through a VCR first and then the VCR to to your slingbox.

As an extra note, the reason you notice this more on your home network vs a remote connection is because the signal on a LAN is less compressed.