hey guys
yeah I have never heard of this happening... which leads me to believe that it has to be something design-related with the outboard circuitry. Not sure I can help much, other than to say that you should have a good look at the power supply. Perhaps have a look at your code as well and see if there is anything that would make it hang unexpectedly.
D
PS: I submitted a question to Atmel engineering to see if flash memory loss is possible under some unknown condition... Will let you know what they say.