I don't think that it's a problem with flicker. I can download gigabytes, having bittorrent running, etc. This never caused any disconnect. LinuxMCE is not just causing diconnects, it completely prevents my dsl router from establishing any connection. When I tell the Linksys router to bring up the connection, it takes some minutes, then it tells me that it could not get an ip address from the pppoe peer. I think that the LinuxMCE tries to use the dsl modem for whatever and interferes with the communication between the router and the modem. I'll do some changes in my network and connect the dsl modem directly to the routers wan port, so that the LinuxMCE has no access to it. I think this will solve to problem, but I'm still curious what the LinuxMCE tries to do.
It sounds to me like it might be that your Core's somehow offering DHCP addresses to the WAN side your Linksys... and if your Linksys is set up to get its WAN side connection from your ADSL providers DHCP server then that would certainly cause a problem like you have described.
Can you explain in detail how your Core/Linksys/network are laid out? what connected to what etc etc would help figure this out I think.