This happended me many times:
I'm playing a level and i turn/open a door/enter in an area, and all freezes. I can't even crtl+alt+del X, trough i can still connect from another computer and kill d2x-xl, and X gets better again.
I tried to record something from it with gdb or strace from the other display(remotely), but both seemt to not give the correct DISPLAY env var and i got a "couln't set video mode to ...."
Seems happening esp. when the number of visible cubes sightly increases.
I can reproduce this bug easily on D2:CS level 2(the door near the first fuel center), D2:Vertigo level 1(behind one of the 1st doors).
I fond something related on my gfx card with this symptoms, and tried it's fix, but it was the only game where i get it and i still get it...