I saw something like it which I identified as being down to excessive memory use by, ironically, the system load applet.

(I identified this by using top from the xtern and hittimg m to sort by memory use)

I remember reading this has been addressed in the latest version of that, btw.

Since then I haven't had it back, but I do tend to close apps when I'm done.

If you get it again I'd recommend taking a look at top or dumping the processlist out to a file to try and see what's going on.