PIC serial going to sleep!

Hi,
I have a board which uses pinsB6 and B7 as a serial port connected via 1k resistors to a PC. Never have any problems with it receiving or sending data.
It is a data logger for want of a better word (it does have other jobs). It reads in data, transmits it to the serial port and also stores it on an I2C eeprom. This is done under interrupt from an external trigger. Otherwise the PIC is asleep (@sleep)
Occasionally after a while the PIC will stop sending data to the serial port although it does send to the eeprom as normal.
The only thing I do on programming is disable the brownout (as it is a 3.3v device).
Any ideas why this may happen with not showing the serial data but still writes to eeprom?
Thanks in advance
Steve