Thanks for your information Bill, and its definitely not OT as I will be using a modded 20D at times.
I have had my suspicions that the focuser draw tube may well be a part of it. Initially I had the SX attached similarly to how you describe, but have now attached it by threaded extenders directly to the drawtube (without any noticeable improvement) . One of the issues I have with this scope is that after focusing and I lock down the drawtube the image moves and goes out of focus. Consequently I have to have the tensioning knob set quite firmly when focusing (using motor focuser)
This is a second/third hand scope and when I bought it (sight unseen) it was not in good condition. I had to send it back to Japan to get the mirror reground as well as recoated. While it was there they informed me that there was a non-standard fine focuser knob attached and that the focuser shaft was bent, as was the rack. If the dollar gets to 1.10 I'll likely put in an order for a FT focuser. Or should I wait until it hits 1.25
Last night after refocusing (not just a touch up of the previous focus but racked it out and back in again) and paying attention to the amount of pressure being exerted by the screw, I took 150 min of NGC3621 with hardly more than a couple of dozen pixels movement in declination. A vast improvement on previous nights efforts!