>What happens is on some of the characters sent out the USART
>the upper and lower nibbles are swapped, so that instead of
>sending out 0x0D it sends 0xD0. What could cause a "nibble"
>swap like this? It does not happen on every character.

This sounds like your interrupt routine services a timer interrupt while you
are in non-interrupt UART handling code, and the contents of the W reg are
not being restored properly. Remember that the common way of
saving/restoring the W reg uses the nibble swap instruction.

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