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Gooday Chris,
This is my understanding of what is happening. Keep in mind that guiding is reactive, the error has already happened and the corrections are compensating for it. Essentially the dog chasing its tail.
When guiding in RA all the guide corrections are speeding up or slowing down the RA motor to compensate for worm/gear PE.
In DEC however the motor is stopped, so when a DEC correction is issued there is inertia, stiction and backlash to overcome. So it may take 3 or more guide corrections before the worm gets the message and actually moves and then the motor stops again. With good PA this doesn't happen very often, with bad PA its continuous.
You get the classic sawtooth pattern looking at the DEC graph in PHD. /l/l/l/l/l/l/
In some mounts there is always the risk that the DEC motor will overshoot and the same situation will happen in the reverse direction.
Bad PA can also effect RA as well, assume the star is drifitng S and the DEC motor is getting a lot of N corrections to move it back. Because of the bad PA the star may actually be moving in a NE (or NW) direction so now the RA corrections have to compensate for this as well.
Cheers
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