I've been fiddling with this for too long now and have had enough of it.
Reducing it's size was blowing square black holes all over the blue as well as other issues.
I got a bit more data at a lower temp and new set of darks and it hasn't done it this time
About 8hours data in 10min subs, 4hours of which is O3.
12" newt, atik11000, OAG.