When operating within the rated voltages of a pic 12F683 does the supply voltage effect the generated baud rate?

I'm struggling with series comms using a 12F683 at 4mhz sending 8N1 data using serout N9600.

I have a logic analyser and it reports the baud rate as 8696 which is around 10% off and I think causing my data corruption.

I have had this working before with a 4mhz clock for months. very odd? Any ideas?