Comment 11 for bug 389686

I see a similar effect here. I had been running with desktop effects disabled (metacity), and then re-enabled them (switching to compiz). it seems that gnome-session is continually respawning metacity continuously, chewing up lots of CPU. metacity fails quickly because compiz is already running, but it gets started again immediately.

I think the problem is likely in gnome-session rather than metacity itself, though I haven't investigated.