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Hi Dom,
What you see is noise. I had the same and it looked like someone went through my nebulas with a rake. A clever man here on the forum enlightened me and told me hiwnto fix it. Never had it again afterwards. What you need is a process called dithering. It essentially means that you take each photo from a slightly different angle. So instead of keeping your scope dead Center on the target you Center each shot slightly different. The exposure itself of course needs to be made with the target remaining still. When you auto guide with Phd there is a function that performs the dither for you.
By dithering, the noise pattern ends up being always in the same spot whereas your target signal moves around. Since stackers like DSS lock on to signal from stars they automatically align the image so that signal sits on signal in the stack. The side effect of that is that the noise is now always in a different spot of your image and whilst the multiple exposure layers you are stacking keep building signal in the same spot it rejects the now randomly occurring noise. Works pretty well.
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