The M1284 seems to have an internal problem where large amounts of serial data can choke the processor.
A capacitor of 100pF on the RX pin to ground can solve this problem.
More info : http://www.mcselec.com/index2.php?option=com_forum&Itemid=59&page=viewtopic&p=60860#60860
PORTC is connected to AVCC and not VCC. This can cause the brown out detection to trigger.