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Pete,
I have the same scope as you and same focuser. I highly doubt its focuser tilt if you have the bottom bearing done up quite firm. Also, I doubt its camera tilt to. But im not saying you dont have any either.
In my experience, this is mis-collimation. It took me a while to pin down collimation on this scope, but i have finally done it using a full frame chip (with some great help from Stevous67).
Also, do you see any variation in the orientation of the stars in the corners when you flip the meridian or point to random points in the sky? I'm thinking tube flexure here.
I found that a really reliable way to get very close to collimation is to defocus a star in the centre of the chip and get the rings concentric. do this for 4 position angles of the camera at 90 deg intervals. as my chip is not square, this was important, there was variation as i rotated. but for you, using a square chip, this may not be such an issue.
Josh
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