Your problem with Scully sounds a wee bit familiar. I have a four-year-old male cat named Yogi, who is suffering from something very similar. It started a couple years ago, when I thought he was chasing a flea around his hindquarters but after doing this thorough search I found no fleas, and none of the other cats were suffering from fleas either. This would happen periodically, usually every couple months or so I would notice it. I mentioned to the vet at the time and she thought it was dust mites, and knowing the age of the building that we were living in at the time it sounded quite feasible. It wasn't till early this spring, that I realized that things had progressed a little bit more with Yogi. He was having these spasms a little more often maybe once a week, and when one of these spasms took effect he would become very agitated with himself and start to dart around the house. This would only last maybe five or 10 minutes at the outside, but sometimes he would take his frustration out on one of the other cats, nothing serious.
A couple months or so back I ran into a post, where somebody was having trouble with the cat that was self mutilating. Some of the symptoms got my interest and I started doing a bit of research into it. I found that involuntary movement was one sign, and a twitching movement of the hide that would give you the impression of having fleas was another - body jerks, excessive grooming, doesn't want to be touched, and the list goes on - but all symptoms don't show in Yogi. His symptoms aren't that prevalent and could be missed by someone who was not paying attention to their cat, and just think that cat was going through a silly fit. Some of the names they have tagged to it is feline hyperesthesia syndrome, rolling skin syndrome, twitchy cat disease, just to name a few.
If this is what Scully has it would be very minor I would think, but it's not to say that it wouldn't get worse over time. I would be talking to your vet the next time you're in to see them and get their opinion. In the meantime, I'll give you a website and you can have a look at it and see if any of the information fits the problem.
With Yogi I have started using some homeopathic to relieve the stress problem and it seems to be working quite well. I find it extremely hard to tell whether he is stressed or not, cats hide stress well making it hard to diagnose and solve problems.
Have a look at this website, it might give you some ideas. http://www.veterinarypartner.com/Content.plx?P=A&A=1998
I would be interested in hearing what you find out about Scully.