In my case the culprit proved to be compiz after all. After my previous message I went back to unity, disabled windows previews and animations and updated my nvidia drivers.
After a few days without incident, I re-enabled animations and 10 minutes ago, windows previews.
Sure enough a few seconds later, after playing around with firing up applications and hovering over their launcher icons my system froze completely, so in my case it was this one compiz plugin.