I understand the issues that you are experiencing with the Slingbox and the Boxee Box. This could be related to the network at home, where the Slingbox is; I noticed that the upload speed is only 500kbps, but the minimum bandwidth requirement is 600kbps for Standard Definition (SD). I recommend you to check the real upload speed at home using this 3rd party web site: http://www.speedtest.net
Using Watch at slingbox.com on the PC, what's the streaming rate remotely? if you get less than 400kbps, probably, it will be necessary to contact the Internet Service Provider to find out why the upload speed is so slow.
Have you tried to power cycle the router and the Slingbox at home? That could help since it will refresh the settings.