psuaero if anyone has any suggestions.
John,
Unfortunately, I have not been able to come up with anything to explain how the polar axis could move one the mount base is locked down --unless the axis itself is moving within the mount body (I assume you have tried to juggle the axis to see if it moves? There should be zero movement.)
One thing that might help is that the next time you do the tests, try to change the guide camera angle so that they are orthogonal to the RA and Declination axes (a couple of plate solves with the guide camera should get you there within 1º).
That might tell us if the field rotation is predominantly caused by some movement in the RA axis or declination axis (or random). Also record whether the OTA is pointed near Meridian or to the horizon (as you know, the "declination drift" is an indication of altitude error or azimuth error as a function of star altitude).
One thing that is not related to your mount particularly is that random flexure caused by OTA position rarely moves the azimuth. But the weight of the OTA can cause the altitude axis to move, if it is not properly locked down. So, make sure any auxiliary bolts (not just the main locking bolts; e.g. things like high/low latitude selection, etc) that are related to the altitude are properly locked -- i.e., when everything is locked down you should not be able to manhandle the mount to move even if you try.
Chen
