Original post states 8" Newt is the primary OTA
Best thing I did to help with this was move to a 9x50 guidescope arrangement. Plenty of threads on it here. MInd you my guidescope was very basic with wobbly plastic focuser. Peter can probably comment on improvement a small refractor guidescope made to his guiding on the 10" too. Good cable management is a biggy too for eliminating guiding noise but probably not likely to be causing the systematic drift described here.
One other comment - its always nice to get rid of any movement, whether flexure or polar alignment or both, but remember too a bit of movement (dither) between frames actually helps with image processing later. Always better if you're chosing when the dither comes in though if possible
