Paul, not everyone can control their AS with the diet alone - indeed, with any one treatment alone. We all find our own way through, usually by trial and error. I'm glad that you've had such positive results with the diet. I also follow a variation of the diet a low/restricted starch diet in that I've eliminated wheat and most gluten bearing grains. I've found that they, wheat in particular, set off my AS something awful - especially in combination with cow dairy.
Have you tried coming off cow dairy to see if there might be a link? My naturopath felt that many people diagnosed as lactose (dairy sugars) intollerant are actually casein (dairy proteins) intollerant. Lactose intollerance is the same whether eating cow, sheep or goat dairy. Casein intollerance is slightly different as cow dairy might be bad, but sheep and goat are fine (different type of protein). Bread and butter, or cheddar on crusty bread (two favourites of mine) exacerbate my AS much worse than wheat alone, but not as badly as if I have butter on corn.
Sometimes it's like being in the Minotaur's Labyrinth. Your AS is the golden thread that tells you you're on the right path.
Hugs,
