FYI: After trying to process another image and failing again (even when I took flats before and after an imaging session), I decided to take everything apart and clean it again to see if I could eliminate as much dust as possible to keep this from happening again. In doing so, I finally found the underlying cause. Samara was right - something was spinning, namely: My filters! I use 1.25" filters screwed into the filter wheel. They were all loose just rolling back and forth in the screw-in threads of the filter wheel.
I took about 800 mostly 180 second of the Draco Triplet and I think it's toast, as I was having terrible issues getting the central galaxy's halo to appear and there's a chunk of dust right in the middle of it. I tried my Fitswork process and it didn't really work. Oh well - my next image should be fine at least! Thanks again for everyone's help!
I'm so glad you were able to find the culprit, it took me at least a couple of months when I had the issue with my focuser, it was subtle, VERY subtle so it was hard to troubleshoot so I'm glad yours was an obvious issue. Don't worry about the lost subs, it's all part of the experience. Good luck and clear skies!