I had this problem too, it's specific to Mandrake not Linux.
Starting with Mandrake 10.0, the default route isn't set up properly.
What I did was add ppp0 as default route in a startup-script:
In /etc/rc.d/rc.local I added the lines
route del default eth0
route add default ppp0
This works fine if you have only one internal network, if by any chance eth0 is gateway to more networks you might have to add them manually to routingtable.
Maybe you don't quite understand what I'm talking about, if my first suggestion doesn't work just return here!