iPhone 4 tethering help

I'm on 4.0.1, jailbroken, and my carrier is Fido (in Canada). We don't have to pay extra for tethering, it's included in our data plan. So I plugged my iPhone into my PC and wanted to test it out, went to Settings and enabled tethering, and it worked. A few minutes later though (usually around 2 or 3 minutes after I've enabled it), my PC has no internet connection (it still shows as connected under "Network Connections" (win xp SP3), but when I go to google.com, for example, nothing loads. I have to unplug the USB-sync cable from the iphone, plug it back in, and I get another 2 minutes of browsing before it stops.

I tried using tetherMe, just to see if it would help at all, but still the same results..

Anyone?