Comment 34 for bug 127101

Brian has hit the nail on the head with this one. As I am pressed for time I will only briefly outline some steps that eventually (after a few minutes) seem to result in this bug:

Run the following two scripts in separate terminals as a regular user:
while true; do totem /usr/share/example-content/Experience\ ubuntu.ogg; done
while true; do sleep 10; killall totem; done;

Run the following script in a root terminal (i.e. one you have sudo'd to root in):
while true; do chvt 2; sleep 3; chvt 7; sleep 3; done

Now a variety of things will happen:
1. You may get the infamous flashing grey block lockup
2. You may find you VT consoles go funny colours but you can still switch back to X and X appears to be fine.
3. You may find that totem suddenly refuses to play videos and looking in dmesg shows something similar to the following:
no MTRR for d0400000,200000 found
mtrr: no more MTRRs available