This took me a little time to hunt down on the internet (and here at Freaks), so I thought I'd post a little synopsis in case others come across the same problem. It may save a few hours of head scratching.
Issue: Atmega1284p uasrt receiver (RX0) causes unpredictable crashes / reboots.
Package in question is 40-PDIP.
Resolution: 10k series resistor in line and 100pf cap to ground.
The information is 'out there' but was not easy to find (or my google-fu was letting me down).
Hope this helps somebody.