crazy (and fake) lags

Hello,
I've had this problem for a while now, and it's really starting to get on my nerves. I consider myself as someone who knows his way around computer, also many of my friends are - and nobody has a clue about this thing.

the example I'm going to give is while using the software from Playchess.com, but this happens on every many-clients-one-server type of software I've used in the past.

this is the deal:
I often experience really intense lags to the main server, 15000ms+, when the connection is actually just fine.
when I connect to another client, the ping is just fine at ~200ms.
also, the network is completely fine and any download I start will go at full speed (1.5mbps).

this ONLY happens in connection to servers, never to another client, and never in websites, online games (ingame), downloads and whatever else.

the computer I'm on is amd dual core 4600+, 2gb ram, so not a weak hardware problem.
the connection to anything beside the server is fine, so not really a connection problem either.
this happens also with other programs and completely different servers, so it's not this specific server that's giving me a hard time.
from every possible scanner (windows defender, spybot s&d) the computer is completely clean.

so I can't even guess what's causing this.
any help will be greatly appriciated.

thanks a lot,
adam.