Hi Daniel and All:
Important update:
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I set up an Arduino that I believe (I will need to 100% confirm early next week when my colleague returns) is programmed with fuse settings as per standard (re Brownout being disabled, assuming that is indeed the standard setting as Daniel suggested yesterday), and with our external circuit in place.
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I then connected a signal generator with sin wave output in about the +/- 10V range, and series diode to give only positive voltage, and connected it to the Arduino round jack power inlet.
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I then varied the frequency over an approximately 5 minute period between about 0.1Hz and 1000Hz....
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And Voila - I erased the programming in the board in a manner similar to what happened before!
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Note that in the past, one time it was the boot loader that got erased (I initially thought I had physically destroyed the USB chip, but reprogramming the boot loader fixed things), and most times it was our program that got lost. This would make perfect sense in ratio since I imagine even a short program is likely longer than the boot loader, and so the chances that the user program rather than bootload got corrupted would be high - assuming random corruption.
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Next week I will try, with the help of my associate, to enable the Brownout Fuse and see if that fixes things. My interpretation, admittedly a bit shaky, is that if neither the bootloader nor our program is writing EEPROM normally (I still hope to have this confirmed the Arduino Powers That Be...), that simply enabling the Brownout Fuse is enough to (theoretically) fix things?
Regards,
David