Basically what I have is a UFO report. I do have a picture of the UFO. But in order to convince anybody that this UFO actually exists, I need to be able to make the UFO appear on demand!

Quite so. Intermittent bugs are undoubtedly the worst. If your own code analysis doesn't find it then more often than not the only thing that will help is to log everything and then pore over those logs when the bug manifests.

As I said in the title of my post, the "problem" only happens some of the time! This is not a "hard fail" and as such, I expect there to be some difficulty in making this repeatable. Anyway suggestions about how best to write such a "beat it up" test script would be welcome!

I ran my SSCCE script there 1000 times just in case. As expected, there was no output from any of the runs. I think that we can safely say that the code there, in isolation, is not at fault and you will have to look elsewhere for the cause of the bugs. Presumably you are on MSWin32 since you mention ActiveState? I can't really help you with automating large runs of the code in that case, sorry.

In your shoes I would slowly expand the SSCCE adding more and more of the surrounding code and running it as many times in a loop as you think prudent. Once you can reproduce the bug (at all, not every time) you can then concentrate on the last thing you added to the SSCCE and try to spot what change that has made.

Good luck with your debugging.