Disconnecting problem

Hey, first of all, I'm new to the forums !
Played TFC for some years as well as Unreal Fortress (yes the UT'99 one)

I've started playing since yesterday but I have this weird problem going on.

I've read all 'solutions' for the disconnect after 1/2 minutes but nothing helped so far.
(Forwarded my ports, reinstalled FF, No firewalls, and so forth)

In some servers I have no problems whatsoever, and I played for over 4 hours yesterday on 1 server.

But on about 75% of the servers I still get the disconnect (upper right corner of the screen: "Warning: Connection problem detected")

I run an AMD 4200+, 7900 GTO, 4 gigs of memory, and a 20mbit connection.

I've tried alot of stuff but nothing resolved the problem.

Any suggestions ?