Connection reset by peer & software Caused connection abort

I am using Ubuntu breezy Linux.

First, i installed ssh. Then i created a user.

I had a couple of friends that would aid me in testing my new sftp server. I told my first friend to download the WinSCP 3.8 beta and to install it.

Ok, so he can connect to my server, browse around and upload files. But, the problem is that after having uploaded for a while, it just stops uploading and his client kinda freezes. So, the upload just crashes. Not after a given amount of time tough, it can crash the first second and it can crash after having uploaded files for 10 mins. The error message he recieved was: "Connection reset by peer".

So i tought maybe there is something wrong with his connection, so i asked another friend. He downloaded WinSCP 3.7.6

He could do the same thing. He connected without any problem but when uploading files, it crashed just like it did for my first friend. His error message was "software Caused connection abort"

I have a D-link 604 broadband router. I have port forwarded port 22 and also opened port 22 in the firewall. My friends told me their IP-adresses but they where not in the firewall log. I dont know where my ssh logfile is.

Can someone help me? Why are the file transfers crashing after a while? Just tell me what to do and i'll do it.

I've been testing some more with my friend. It seems this does not only happen on sftp, it also happens in ftp using vsftpd.

He connects and starts the upload. The speed is going very much up and down all the time. Like the transfer isnt stable at all. Sooner or later the transfer dies and my friend has to reconnect.

I've tried on my other computer as well, and the same thing happens there.