Does this happen right after a reboot, or does it start happening after a day or two? There was just a thread, started by me, about this that just just off page one now. Apparently there are at least a few of us with this issue after it's been up for a while. A reboot fixes it for me, but only temporarily.
FWIW, my core/md hybrid is an i386 install on an AMD dual core, with 2G ram, and an nvidia 7300 SE (cheap-o) video card.
I just rebooted and all the video works ok. It sounds like it is the same problem. Are you having any other problems using i386 linuxmce? Have you used the amd64?