Terry, I'm starting to experiment with flats, I'm not convinced it is the cause of this oscillation though, although it could be. Because I'm doing all this with a dob the target is moving all over the place in the frame, but the oscillations seem to come out at the same frequencies. So I think it is probably either the camera response or something inherent in the star analyser, or both. There is a thread on the astronomical_spectroscopy yahoo group currently which seems to suggest that a grating may produce this kind of effect.
My main ailment which I'm trying to cure with the flats is a bayer type pattern from my QHY5, see the top right of the attached screenshot. Even though it is a mono camera it still exhibits a bayer style grid. I haven't been able to find an answer as to why it does this. I think possibly the chip is the same as is used in the colour QHY5 and has a different gain based on whether it would have been R,G or B. Not sure. Of course, the real answer is to get a proper camera, but I have to make do with what I have
Ken, background sky removal I hadn't even thought of. I'll add it to the list!
Thanks for the help guys, it's really appreciated.