Since the skin and fur seems healthy and you are treating for fleas, etc., I think that this is probably a nerve or muscle issue. Especially since it is occuring exclusively on the back.
It could be as simple as a pinched nerve. There are a lot of possbilites here, not necessarily serious. When you can, if this continues, I'd have it checked out by a vet.
In the meantime, try Yucca root supplement, which has anti-inflammitory properties and make sure your cat is getting enough hydration (canned food along with wet and drinking water) - I don't think that dry skin is an issue here, but it's best to make sure.
Also, I'd cut out the baths (for all your cats, actually). You aren't doing it frequently, so it probably isn't an issue, but shampoos dry out a cats skin and strip necessary oils and bacteria (yup, necessary bacteria). There's no need to wash kitty, just wipe his fur with a damp cloth if he gets into a mess, otherwise he'll be fine on his own!
Good luck. Keep us posted.