ATMEL 169PA resets with MCUSR = 0
Answering your question directly will require a lot more information. How do you know it's restarting? Have you accidentally cleared it before it's read? 

Otherwise this would indicate a Power-On-Reset where the voltage drops below VPOT (as specified in the datasheet). I smell a hardware issue somewhere.

[Edit] Given the new information that the unit is being subjected to very cold temperatures. It's almost certain that some units are likely failing because of this. I'd contact Atmel with the failing chip information (lot #, date, etc that are marked on the device) and ask for more guidance. Perhaps a lot had issues. The failure rate seemstoo small to be a PCB issue, but could definitively be explained by some devices not meeting up the -40C spec.