2 possible solutions
1) Reboot your Slingbox 500 by unplugging it for at least 30 seconds, the powering it back up again.
2) I've owned and used for 1½ months a Motorola SBG6580 cable modem/router.
With Comcast in South Florida, the internet signal was very unstable and the device ''died'' on me, after 6 weeks.
After 30 days, the retail store didn't want to exchange it.
I had to send it back to Motorola in Pueblo, Colorado (I had to pay for shipping).
I installed a Comcast cable modem/router, because i need Internet.
When I got back a replacement Motorola cable modem router after 10 days, I sold it for half price.
I kept the Comcast which works flawlessly.
Motorola SBG6580 is a piece of garbage.
The network settings you are using could interfere with the Slingbox from leaving the network. The Slingbox uses DHCP to configure to your network, not a static IP address. I would recommend enabling this setting on the router, and resetup the Slingbox after doing this.
Here is an article that can further explain this:
Sling Media Moderation Team