This seems to be an issue with the router itself and not the Slingbox. I suggest that you reset your router and configure it again and also Restore the Slingbox to factory settings with the help of this link,
Let us know how it works.
Good news to report. My slingbox is up and running without issue. I eneded up replacing my router by upgrading to the Linksys E3000, configured the ports for the slingbox and everthing started to work normally. I also replaced the powersupply just in case with a genereic $24 Radio Shack setup.
We use the slingbox every other weekend at our cabin and it has been flawless since the upgrade. It might be time to try the HD version.