Thanks for the help with BF3 (changed the ports to the ones I really need (was set on all,... weird), and made an exception for BF3 for the A/V), but the problem still occurs, at least I can play lagfree and perfect for the first 5 ton 10 minutes.
Of your list, I have played the following within the last three months:
Counter Strike
Team Fortress
Minecraft
TrackMania
Borderlands
Killing Floor
Quake
Tribes