You're not the only one Allen. My scenario is identical to yours with the exception of the service provider and location. I'm in Coral Springs Florida and I'm using Advanced Cable Communications. But we are using the same boxes (Slingbox 500 & the Arris (Motorola) DCX-3510M).
When I called Slings customer support line, because I'm still under warranty, we tried everything they could think of and we couldn't find a solution.