problem uploading files

[Luis Gustavo Madrigal Salazar <tavo@scratchy.emate.ucr.ac.cr>]

I recently installed debian 2.2rev2 without any graphical stuff, but also
I installed mason, junkbuster, and ipv6 just for testing.
When I connect to my ISP, I can download files without any problem, but
when I try to send a file bigger than 3kb(using ftp, or send a mail with
an attachment) the connection freezes (or it proceeds too slow: 2kb/10min)
I have tried stopping mason, junkbuster, and also disbling all the options
the networking script sets (like syncookies and some antispoofing flags)
but problem persists. Does anybody have an idea? (I've also tried with
other ISPs and problem seemed to be my box)

thanks!