If it gets worse in longer subs, it could be a mixture of tilt and focus shift..
Also, with a newt, as the target gets higher up, the focuser is holding the load out horizontal.. this induces more and more flex on the drawtube...
A motorized moonlite will fix the problem!
If you're looking at motor options, go for the stepper motor with the mini-moonlite controller... Thats the way to go on an imaging scope. The DC motor will do the job no worries, but it does not have absolute positioning, Ie. The focuser does not know the current position of the drawtube, and as such, you can not save your "focused" position and simply return to that spot night after night.... The Stepper is a little more expensive, but worth the coin!
Alex.