This problem appears two/three times per month, and then disappears with no reason.
After starting system, my internet (LAN works fine) connection begins to slow down. Usually I have about 15 - 25 kb/s. And minute or two after I start my Debian it begins to slow down, and after 4 - 5 minutes I have 2 - 3... Addictionally, two strange things happen:

-the diagram of the transfer is regular: tens seconds of 0,5-5 kb and then very short jump to 10 - 20 kb. Then - dozens of seconds of 3 kb transfer. It looks like that: [2; 0.4; 4; 2; (....); 3; 0.2; 15; 0.6; 2; (...)12...] I don't have own website, so I can't put a odd screenshot from gkrellm there, but if someone is interested, I will send it via e-mail (5 kb).
-If I close for some seconds all programs that use connection, and then start them, all starts from the beginning. Transfer is good for a while and then gradually slows down...

In the same time mates from my LAN have good transfer. When I change to WinXP (i have both, Linux and Win) I have normal transfer and it doesn't slow down. When I back to Debian - it begins again. I had this problem before, it lasted for a half/whole day. And next with no reason all was allright. This time it has been going on for two days and I don't know when it ends...

Somebody knows the answer ? I will be very gratefull.

P.S. Sorry for my english. If some parts of my post are not uderstandable to You, please write it. I will do my best to correct them.