Bawapp
I have read your posts - 1) just can't remember if you are using pellets or loose powder 2) are the primers going off?
The remington standard 209 primer is a stronger (hotter)primer than the ML primer - so I am not thinking that going to a lower temp would help. I use even a cooler primer the Remington 209-4 for 410 shotguns - It has never failed me but I am using t7 loose powder. In the colder temps the t7 pellets can be a bear to ignite. A lot of people are using the Winchester 209 with really good luck with pellets - might try them. Then there are even hotter 209's when you start getting into the Federals.
If you are using pellets, how old are they and are dry, have they been exposed to moist air for any length of time - they will absorb it?
Trying to remember your accounts - I would really be leaning to the moist powder end of the spectrum... just my 2 cents