Gday Houghy
Whilst i have no experience with the EQ8 mount/hbx etc
what you are seeing there is exactly what we have been looking at
recently with LX200GPS scopes in polar.
I dont know if you have ever played with an LX200 GPS in polar
and manually operated the DEC slomo ( do it carefully
and with a very light grip ). The scope knows where it "should" be pointing, and if it gets too far off track, it arbitrarily returns to that spot, and it does it very rapidly.
We havent fully tracked it down but we do know several things can happen.
First guess is something is going on where the scope keeps 2 ideas of where it is pointing. Your scope is drifting in DEC and at some point, the system may say, whoops, too far off target, and then just arbitrarily slews back, which invokes backlash application which creates a jump???
Just a thought
Andrew