AFAIK Belkins are better port forwarded than run in UPnP mode. Have you checked to see if there are firmware updates for your model of belkin? Firmware upgrades can sometimes fix some issues. But if there is a firmware upgrade, check for user feedback to see there's no bugs with it. Main, / UK Support
I found this page http://www.belkin.com/support/downlo...5D5230-4&mode=

Are you sure the problem is your belkin & not the modem? I found this.
Are you using USB or ethernet between your devices & computers. I've read where USB can struggle for high usage. Such as discussed here.