We had a lot of problems with the Sportsters here -- turns out
that they had a defective ROM.  (We are supposed to be getting
new ones).  I have a Megahertz PC Card ethernet/modem, and it
works (but it is also US Robotics).  Motorola makes really good
modems, but they are more expensive than average.

http://www.carabelli.com/dentist/lore/usrincident.html

 
Anyway, I got mine to work by Forcing ARQ  (I forget the
init string), and DISABLING compression.  Performance 
sucks, but it works.

Kevin