I'll add a little here, I have the same problem. FC6 latest kernel, latest bios, multiple m2n-mx boards. Once the link goes "broken", I can get it to come back up if I do a;
rmmod forcedeth;modprobe forcedeth
so have used that as a workaround with a ping test in a cron job. ifdown/ifup has no affect, ifconfig acts like the eth0 is up.
Also, I have win2003 on an identical hardware setup with no errors/problems on the link. So it looks to me like a driver issue. I've been trying to compile 0.62 but no luck, I get an error and warnings - one warning of great concern since it seems to be dangerous to me. I've posted a seperate thread for the compile problem, might be useful but even if I get it compiled it might not resolve the problem.