I used to get it in connection with Iritis every few months not all that long ago.
In my case the doctors finally decided that it is a manifestation of an autoimmune problem, likely linked to RA (which virtually every male and some of the females in my dad's family have problems with sometime during their lifetime.). In my case it is an extremely painful problem which has caused me to go into the darkest spot I can find in the house during the day (I go into the bathroom, turn out the light and close the door, get into the bathtub and draw the shower curtain).
As I wrote, the doctors finally figured out it was an autoimmune condition connected to RA, much like Sjögren's Syndrome. But different in that it attacks the eye at the iris instead of at the tear glands, as in Sjögren's. Since I went on suppression therapy, I've only had one attack over a year ago. You might want to ask your ophthalmologist if they've considered the possibility of autoimmune problem. Considering the repetitive nature of the attacks it might be. In that case, a rheumatologist might be a good one to consult with.
Forscher