Hi,
I don't know much about I2C but I suspect that the issue you're having could be timing related. If you're using the PBP commands for I2C then then the communication is bit-banged and software timed. Darrels SPWM routines uses interrupts to do its thing which will disturb the timing of the I2C communication.

I'm curious what the issue with using hardware PWM actually means. Since the hardware is built to use the CCP modules I'd do everything I possibly could to actually make use of them instead of reverting to bit-banging PWM.

/Henrik.