I would make sure that the Slingbox is streaming over TCP. To make sure that your Slingbox is streaming over TCP, you could try creating a port forwarding rule on your router for the Slingbox.
These articles should assist with setting this up:
- Slingbox.com - What is Port Forwarding or Port Mapping?
- Slingbox.com - What's port forwarding/port mapping?
Hope this helps!