I may be barking up the wrong tree, but -- how do you clean? It may be that you're not getting it really, squeaky clean, and sometimes it seems that the merest trace left in the bore will "attract" more.
I'll also say that I own lots of rifles, and most of them will begin to show a bit of a downhill slide in accuracy after 50 rounds. If I'm going to the range for a high-volume shooting session, I will take along what I need to give the bore at least a partial cleaning somewhere along the way.