I've had a similar thing with one of my cats and the more I've dug, the more I've found out that there is usually an underlying medical condition causing the overgrooming. Sometimes it can be stress, so if there are any particular stressors you are aware of, it could be that, but I think diet would be a good place to start.
After about 2 years of thinking that Milly's was stress related due to a bullying neighbour's cat, she started getting worse and a blood test revealed elevated eosinophils. Started hunting around my brain and realised that it had first happened when royal canin had changed their food from outdoor mature to a 7+ and 12+. And the main ingredient in the 12+...wheat.
Through a process of elimination and ending up on a raw food diet, we have worked out that all there years, the overgrooming was simply skin irritation due to dietary intolerance. There doesn't seem to be one single trigger as she also reacts to some grain free foods such as james well beloved and orijens, and also the hypoallergenic food! Diagnosis was 'confirmed' when I went away for a few days and oh forgot to defrost a new block of raw food, so had fed her some hypoallergenic biscuits. When I came home, she had pulled all the fur off her hip again. It only seems to take a few mouthfuls to trigger quite a severe reaction, and the itchiness lasts for weeks. She also had Piriton for a while and it helped initially, until she became intolerant to one of the ingredients - probably either the maize starch and/or lactose, and the itchiness rose again.
I would advise trying a change of food to something raw, hypoallergenic or at least grain free and see if it makes a difference. It might take several weeks. I would also advise a blood test to check for raised eosinophil levels, which might give an indication of allergic response and maybe chat to your vet about antihistamine trial. If it could be stress related, a feliway/valerian plug in might help.
There are other drugs that can help but need to be used with caution, like steroids or we even discussed atopica at one point. It would have solved it I'm sure but it would really only have been masking symptoms. It's taken nearly a year of faffing with diet but for the first time in about 3 years Milly had full fur on her side.