G'Day Greg,
So you are taking these shots on a fixed tripod, not tracking, correct?
I think you will find this results in too much drift between frames for registax to correctly register Saturn. I think it might be registering (aligning) the background noise.
You could try manually aligning saturn i.e. select manual aligmnment, single alignment point and go through each frame and select Saturn in each frame. There is an option on the alignment page to select the amount of zoom.
Good luck... you are really pushing the limits!
Al.