I'm building what Winchester SHOULD have done when they created the WSM series of cartridges, I have appropriately named this wildcat: .243 LASER. In essence, it is simply a .243 WSM.
In my opinion, Winchester missed an opportunity to create a superior ELR cartridge when instead of following...
My thought is that you are at the top end of velocities for your bullet/powder combination. Even if an occasional round doesn't stick, others do. I'd drop down to the next accuracy node.i think you're running a bit too hot.