Turning off Virtual Memory seems to have cured it, for now anyway. Many thanks. I also turned off the memory check in the Memory Control Panel, which cured another problem whereby the machine occasionally stalled with a blank screen and needed the three finger salute to restart.
What is the deal with VM in this case? I have never had trouble with it on a ppc before.

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It's probably an extension or control panel that crashes in the presence of virtual memory. By turning off virtual memory, you make the problem disappear.

Try disabling all control panels and extensions in extensions manager (Except Memory control panel if given the choice), then restarting with VM ON and see if that also cures the problem. If so, you can narrow down a different culprit and keep virtual memory.

If you haven't done this before, typically you add back half of them at a time. If it works, you know all of those are good and the problem lies in the other half that is still disabled. If it doesn't work, undo what you did and enable the other half. Then simply proceed to enable half of the ones that are left and repeat the process, dividing them all by 2 over and over until you figure it out. It's called a binary search, or in Mac techie terms, a "Split-Half Search".