We have just put in a Netgear DG834GT.
without port forwarding viewing over the internet runs smoothly at about 900Kbps. When port forwarding is set up on the router (port 5001) the stream slows to single figures (effectively freezes).
Problem is we have a sling catcher so port forwarding is vital.
What you can do, is to set the rule using a different port number such as 443 and use the TCP as the only protocol.
Also, connect the SlingCatcher directly to the router just to discard a wireless connection as a potential issue.