upload drop connection

[Toba]

Hello the list,
Since I'm using adsl with my ubuntu, I get my connection dropped all the time 
I want to upload something bigger than about 250k (mail, ftp, scp... all 
port). So far, that was not a problem for me as I often use my connection for 
remote ssh and download. But today I really need it for upload of big file and 
aaargh!

What I did is to change MTU to, 1458, 1400, 1492. No big result. I tested by 
sending 4.8Mb of mail but no success after 75% of transmission: connection 
dropped. Default MTU is 1500.

I have dual boot: Under windows, upload works fine. Alas :-(

Then can someone help me? What to do? Has someone experienced such problem?

Specifications:
- Ubuntu version: Ubuntu 9.04 \n \l
- Kernel: Linux sibo 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17 01:57:59 UTC 
2009 i686 GNU/Linux
- Network card: Realtek Semiconductor Co., Ltd. RTL-8139/8139C/8139C+ (rev 10)
- Default MTU: 1500
- ADSL Modem: Prolink Hurricane 5200
- In modem configuration, I see MRU: 1492

I hope someone can help me to get it solved.

Best regards,

Toba