I've been running Linux for 15 years and am stuck. Help!

On Ubuntu Desktop, sometimes (usually after a few minutes - sometimes not at all until reboot) all file write operations seem to hang, while opening files is not a problem. Can't kill processes, they just go <defunct>. Extremely slow drawing of windows.

What's going on here? Memory and CPU look good. Open files are not crazy... io load is very low. Not virtualized.

Any ideas?