sure the bullets aren't getting setback into the cases under recoil? that'll run your pressure up quick and push a primer... How many times have you reloaded that brass and did you ever "uniform" the pockets? might be they are just getting loose or you have a batch of small diameter primers... just a few things to think about...

i ran up some hot (what i consider to be near max from book data) Reloader 15 rounds in the 35 whelen AI last weekend... and i wasn't seeing flattened primers which surprised me... my shoulder and the headache from recoil told me different.. further testing to follow....

BTW $503 on auction arms...