I've tried many Linux distro's, all have had this problem. Currently, Mandrake, since it is easy for me.

My cable conenction is configured via DHCP, and it is set up to use DHCP. My NIC is a D-Link 528CT, an NE2000 clone, using that driver as suggested by D-Link.

Now, after say, 10 or 15 minutes of use, I lose my Internet connection. I can only get it back by unplugging my modem, then plugging it back in, which is kind of like the manual way to get an IP address from my ISP.

I really wish I could get this figured out.

this seems like the power on your line is not up to snuff, it shouldnt have anything to do with your end of the connection, call up your cable provider and complain about your problems then ask them if it could possibly be that your line is being lost due to low power, they can probly tell you from there if its a possibility as this happens most often on the fringe area of their range, they can then send a person out to test how much power is on your line and hopefully adjust it, at least thats what has happened to me in the past

It works fine under Windows, though.

then you should not need to touch the modem at all, next time it happens type ps -A and look for the number next to dhcpcd then execute "kill thenumberyousee "
then type "dhcpcd eth0"
and see if your net connection comes back

Didn't work. However after replugging the modem, and performing the command "dhclient eth0" as root, it came back on.

this confuses me greatly...your net connection should not be going out only in linux and not in windows and requiring a reboot of your modem, are you running through a router? a home network? or just straigh connection to the modem?