I've only been booting into classic gnome desktop since 11.04 and since upgrading to 11.04 I'm having horrible window flickering and what seems to almost be gdm restarting. Maybe it's not gdm restarting but what happens is the desktop will flicker, all windows from all workspaces will come to the current workspace, often all window decoration will be gone, artifacts, and super choppy window dragging.
Anyway, does unity-window-decorator have something to do with it? Is this compiz? Can I replace it with something else?
EDIT: I'm pretty sure it has something to do with the buggy nvidia drivers. I know that the switch to unity-window-decorator is new between 10.10 and 11.04 so thought it may be an issue too.
EDIT:For reference if it does have something to do with the drivers, I'm using nvidia-current on -
product: G92 [GeForce 9800 GTX+]
vendor: nVidia Corporation
I run $compiz --replace & but that doesn't fix it.
Unity-window-decorator is definitely not your problem, as it is almost identical to the gnome-window-decorator. The most likely cause of the problem is that you (probably) have buggy drivers (probably because of your hardware, which you should add to your question by the way), causing compiz to crash or restart. (By the way this is not gdm restarting, that would take your entire Xsession with it).
If you are pretty convinced (though I am not) that unity-window-decorator is the problem, you can change the compiz window decorator to gnome-window-decorator in ccsm.
Also, you can try using metacity (you will lose your effects, but at least it will work) instead of compiz.
See this answer: How to Get nvidia driver working properly ( running experimental 3d support)