Gremlins changing my network preferences from modem to ethernet

I am still on dial-up, and use the internal modem on my G5 (PPC, OSX 10.4.11) I have checked 'use internal modem' on network preferences and moved it to top of the list, with ethernet at the bottom. I also click the lock. But every time I try to connect it disconnects after a few seconds and when I open network diagnostics it shows that it is using ethernet. I change that to internal modem and connect again only for it to hang up again if I open Mail, but if I open Safari and select a website it is usually OK unless I pause or put it to sleep, then it reverts to ethernet. I've never used ethernet so why is it doing it? Is it a modem problem? Would using an Apple USB modem cure it? Or is it my settings? I'd be very gratetful for any suggestions as it is so frustrating. Should I delete ethernet from the list?: