I've got the same modem and had the same problem, i got a sort of workaround from a friend :
open a shell : login with : su (then your root's password)
$ ifconfig eth0 down
and then it might be ok for that session at least...
so the problem does persist, why having to do this everytime you booted the computer ? how get it right ? At the same time some KDE program's are quite slow to start (kwrite, home icon on desktop) but not openoffice or xpdf for example. this friend tells me it could have something to do with the speedtouch... he thinks that this slowness has got to do also with the fact that KDE has to search for my computer to be able to open the programs. Well, I keep on searching. In meanwhile I am connected and hopefully you also.
--news from the slow problem :
as root edit
HOSTNAME = somithg-in-letters
at HOSTNAME there was written my DNS server number