[rmuhlack (Richard)]

Pat, I had a go at adjusting your image using Pixinsight, just using the original jpg image you uploaded in your first post.

The gradient still remains to some extent (even with very heavy handed gradient removal), so my guess is that stray light from somewhere is affecting the lights, which was then not present when you did your twilight flats.

Did you have the viewfinder covered over during capture, as light can "leak" back into a DSLR via the rear viewfinder over long exposures. Other options could be your computer, lights on the mount, outside lights from your neighbours or the street etc etc. Perhaps even a dew problem?