Hi Tom
It sounds like it could be your ISP, Comcast can be known to throttle connections at certain times of the day/night...
Comcast Blocks Some Internet Traffic http://www.gnutellaforums.com/312513-post92.html http://www.gnutellaforums.com/284787-post70.html
Although that being said, your Belkin Router does have a Nat Firewall within, so Port Forwarding may do the trick, it's worth a try anyway...
These are your instructions for your Belkin .........
Port Forwarding for the Belkin F5D6231-4