Roger,
I too get the feeling that S-Big are onto something when it comes to auto-guiding long duration, long focal length imaging. No that I have transfered all my gear off Celestron dovetails and onto much sturdier Losmandy bars throughout I'd say rigidity has at least doubled - and pointing is all the better for it. MaxPoint confirms this too!
What I do wish was better analysis of where I am loosing the auto-guiding war. My thoughts are review the PHD logs looking for either:
1) sudden, large movements in RA
2) large movements in RA that then reverse shortly thereafter
If I can see a pattern I can work to posit a cause and effect. One thing I do want to try with PHD is shooting say NGC 2070 on the same guide star for two full worm periods (say 16 minutes) with the frame being taken every evry 0.5, 1, 1.5, 2., 3.5, 5 and 10 seconds - so after these 7 runs I'd like to see what sort of trails I get - where sweet spots occur and what sort of errors I get chasing seeing.
Much to learn here!