The same type of thing has happened on my machine, but only on rare occasions. It seems most frequent when I've been using the unit for a long time (several days) without a reboot.
Now I shut my machine down and do a cold-boot before any long Medieval playing session, which flushes any garbage out of memory. GoofBall's recommendation is spot-on, so remember to CTRL-S before hitting ESC back to the menu.
If those tips don't reduce the problem to a minimal level, I'd say it's symptomatic of a more significant computer issue - and that you're due for some intensive trouble-shooting.