I mostly agree with you on the extreme powders, except H1000. H1000 is considered one of their extreme powders isn't it? Varget has worked well for me as has Retumbo. I know H1000 is popular but I hate that crap.
The reason I assume that it is a lack temperature "resistance" causing the issue, is because I could think of other reasons, but they mostly do not pan out. I have considered increased fouling when shooting long strings, but I guess I should think horses when I hear hoofs instead of zebras.
