My cat has overgroomed for over a year now. Mainly on her stomach, back legs and (to a much lesser extent) back of her front legs. It's odd, because she over grooms an area to almost the skin and then it grows back, and then she goes again a few weeks/months later - it's almost cyclical.
She saw the vet early on, who said it definitely wasn't anything to do with her skin (ie no irritation, mites etc etc), and there has never been any indication of sore skin. We tried different types of food, but that made no difference (and she's a fussy madam anyway, so that wasn't very helpful).
The vet (x3, as she's had it looked at 3 times now!) is adamant that it's stress related. But we can't pinpoint what's stressing her. She is an outside cat, so we can't keep her in (that is very stressful for her), and if it's something that's outside there's very little we can do about it (apart from giving her the option to stay inside the whole time, which she can if she wants to). She did have a short course of steroids which worked a treat (in that the hair grew back, and she stopped over grooming), but after a while she started again, and the vet was reluctant to prescribe them longer term when there's not actually anything wrong.
We were just advised to use Feliway and try to work out what's stressing her. We do have Feliway (although not all the time), and we think it's a neighbour's cat in the garden which bothers her, but nothing we can do about that.
We, and she, just live with it now. Keeping an eye on it, and making sure she has lots of the things she likes to make her feel happy (laps, pillows, company...)