Short answer: Turn of DHCP on your interfaces.
It sounds like you have your cable/dsl modem hooked up to the "wrong" interface.
That is when Mandrake boots it's obtaining an IP for the local ethernet card and correctly changing the IP's to reflect the information given to it by the ISP.
If your Cable/DSL modem is on a shared network, then it's likely that you have the cable/dsl modem and your workstations on the same "level" which is not correct.
That is your machines and DSL/CABLE modem are hooked into the same hub with no firewalling or NAT.
In this case you shouldn't really do this.
DSL--------NAT ROUTER's WAN port ---- PROTECTED LAN PORTS---- MACHINES
But you may have
DSL----------- HUB --------- COMPUTERS