screening / glitchy operation - sanity check

[Jonathan]

This is most likely to be a software problem. In what language have you programmed the PIC? If you don't mind posting the code then I'll have a look at it for you. I expect it'll be the PIC not outputting pulses with a smoothly changing frequency, e.g:
Instead of,
|_|_|_|__|___|____|____|____| ...
Something like,
_|_|_|____|__|___|____|____|____| ...

This can be caused by things like stopping the output (briefly) whilst calculating the period which would introduce an additional delay. Are you using the hardware PWM module, or doing it 'manually' with a loop?