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Hi John
Wavytone was correct, 9m FL is very unforgiving. The drive is definitely a Losmandy Digital Drive (DD) but it appears to use belt from the stepper to the RA drive.
I can see the rocking in RA. I don't really think this is a major problem unless directly over head. Either side will bias the drive with mass of the scope thus it is either driving against this slop or riding it down. Either way when guiding this shouldn't be a major issue in the same manner bias weight is used on a GEM to hold the worm against the RA gear.
I also very much doubt the Digital Drive system is the issue. Its rare to impossible for crystals wander off their designated frequencies these days.
I suspect King rate will help apart from that it might be slipping around the belt (is this belt toothed?)?
Putting a voltage regulator between the DD or between the DD and RA motor will achieve nothing. In fact it will interfere. The DD needs at least 12V to operate changing this voltage up down achieves nothing as there is an onboard voltage regulator.
The RA motor is a stepper. It works by receiving a series of 12V pulses that makes it "step" fwd (or back). The DD will count these pulses so retain positional accuracy. Reducing this voltage will make the motor "cog" that is misstep or stall.
So back to the issue...differential flexure as Wavytone stated. More than likely it is this. The mass of the truss at the secondary and focuser twists the structure when pointing one side (east) and twists the opposite way when pointing the other side (west).
This flexure is small but at 9m FL......is amplified greatly.
You best bet is image capture when the object is at zenith which will minimise flexure errors.
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