Ken, I have raised it with Starlight Xpress and am waiting a response.
On what may be a related issue, I have noticed horizontal banding on dark frames. These are barely visible normally, but very pronounced when stretched. I raised this with Starlight Xpress, and they asked me to try again using MaximDL rather than SGP. In MaximDL the banding does not appear - so it looks like it could be an SGP-related issue. Here is a Dropbox link to a dark frame from SGP:
and here is a stretched JPEG from the same frame: