I have an ATI 4850, and I've been struggling for months with random crashes in L4D2, both in CrossOver and in straight Wine.
This morning I finished a full versus campaign and played survival for a couple of hours - all without crashing once, through multiple restarts of Steam and L4D2. I was researching fglrx options when I ran across someone who suggested to look at the output of aticonfig --help. In that output, there are several advanced settings documented. I started methodically working through them, one at a time, to see if they had any effect. The third one says this:
--sb, --signal-block={on|off}
Enable/disable signal blocking. Disable this option when debugging a multi-threaded OpenGL application.
Using that option (as root) caused the xorg.conf to be rewritten, and this options was added to the Driver Section:
Option "BlockSignalsOnLock" "off"
I had to restart X to enable that option. Playing with it enabled, I still get hiccups in-game - but they no longer hard-lock the entire system. I even had a series of three hiccups, and recovered after only a second or two. I have no idea what this option does, or why ATI would choose a default that can hard-lock the user's system. In any case, I'm now fully able to play, though performance is still an issue.