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Hi Charles,
As I have the same scope and mount, but different camera (ST11000M) I may be able to help.
First try guiding with Dec corrections turned off. If your polar alignment is good, then you should not need to guide in Dec. In fact with the MX, a big model, PEC you may not need to guide much at all, though in practice guiding always gives better results.
Try taking shorter exposures, a couple of minutes should be enough to show any Dec guiding problems. Keep your eye on the dec error in CCDSoft (or whatever you're using to image with), if it's not much and you still have oval stars then the problem is more than likely optical.
Are you at the magical 6" back focus, if not the off-axis stars will suffer. Can you email me one of your subs to look at?
I have found with this scope, if the collimation isn't perfect you get oval stars in one area of the frame and nice round ones elsewhere. I have started to think about going for a smaller CCD as the edges of my FOV are really difficult to get right, I think that the STL11000 chip is pushing the corrected circle for this scope.
Cheers
Stuart
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