Getting DC all the time

Hello,
I have such an issue: (sometimes) When I log in to the server, I play for like 2-5 minutes and then I'm getting DC(?) just like when server is going down. I need to alt+f4 my client and log in again. But then the situation repeats constantly, so basically I can't play anyway. I noticed that it happens when I'd like to play during the day, when server is full. When I log in at night, when there are about 5-7k people on the Icecrown realm, it doesn't happen at all.
I checked my network connection, checked server uptime and it is not their fault.
Can anyone explain me what is happening and/or give me some hint to get rid of this issue?

Alkus