OK - just to clarify; I removed the code and tested it with the program as you see it - with the same results. (the other code was mostly to do with altering the period using jumpers)

As I understand it, each command takes Fosc/4 except for goto and a few others I haven't used, so I wouldn't have expected an error of more than 1 - 2 us in the main loop.

I had wondered about the oscillator. I have a few 10 MHz oscillators but can't get them until Monday now, so I won't be able to test that theory until then.