If you reduce the timebase of your analyzer so you can capture an entire set of the FOR-NEXT loop, you'll see that it's not jitter but the increased time when the FOR-NEXT loop exits and the GOTO loop occurs. You have a few extra cycles there to do the GOTO and the exit/initialization of the FOR-NEXT loop so every 16 iterations you get a small gap.

The code I posted takes advantage of the fact that incrementing a byte from 255 just rolls over to 0, so it has the same effect as your FOR x=0 to 15 loop without the extra loop involved.

Obviously, your pic can't do anything else but this, otherwise the timing will change. Your other option would be to setup a timer and do this in an interrupt routine but you won't be able to get exact timing with that since there can be a slight difference in the timing of when the ISR runs.