I haven't seen it, but there are a couple of things to try.
Is this only when playing a DVD? Or only playing an AVI? Or only playing something else? If it only happens when trying to play one media or format type, try something else and see if it still happens (fyi: mplayer can play pretty much ... anything.
If it happens with a lot of formats, turn off xdm (by booting to a text login), startx, and get it to happen. Then, copy your /var/log/XFree86.0.log to somewhere other than /var/log (like your home directory) and post it here.