Bawapp
Well that answers both of my powder questions... Now I am a dumbfonded also because a Remington KleenBore 209 really should start that setup.
When you pop a couple of caps before loading do you see sparks come out of the barrel or if you shoot it close to the ground it should move things - dust - dirt - leaves?
My next question is the powder getting contaminated inside the bore?
grease from the breech plug? oil from the barrel? and/or condensation in the bore running down past your PB and into powder? These are all way out there but just thoughts.
Next question working along these lines - if you were to load and shoot right after loading would the gun go off? Is your problem occuring after and extended time in the field?
Of course there is a possibilty the primers could be weak - can not imagine that but it is possible.
I wasn't much help I'm sure but these are my thoughts...