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Hi Doug,
I used to have that happen to me sometimes. If your polar alignment is a bit off then even though you are autoguiding and the centre stars in the image stay aligned the outer ones will rotate.
So first off you should check your polar alignment and chances are you will find it was off to some degree.
Secondly CCDstack has an excellent plugin for high accuracy alignment. Once I started using this I found that sort of alignment issue a thing of the past. It costs US$99 but it is worth it. Just one click and it does it all automatically.
What is happening is your polar alignment is a bit off so there is rotation between images as well as any slight drift. You can do it manually in CCDstack and you'll have to move them on top of each other and then rotate them until they line up. The plugin does this automatically.
The 1.6X extender is a nice accessory isn't it? I've been using one lately as well.
Greg.
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