If you're running from the internal clock reference, check the frequency drift vs. temperature curve in the datasheet. Looks like it varies by maybe 0.5% over 20 degrees C, which could be enough to take your baud rate out of spec.
The one time I've gotten a crystal-referenced MCU enough out of spec to be a problem, I had it at about -190 C, just above the liquid level in a dewar of liquid nitrogen.
On the other hand, the first time I used Processor Expert for clock setup I accidentally selected the internal reference instead of external. My first prototype worked OK but without trimming it was hit or miss on the others.
Scott
