the heartbeat in the crosshairs
And this is one of those times that once seen, can't be unseen... it is a line that once crossed...
I too am one of these folks, although it is more akin to "snow wrestling" for me Art!, that likes a group of bullets to make a little tiny hole at longer distances.
Yup, have someone else give it a go with your set up and see what you see. Before you start playing with "stuff", see if it is the "stuff".
And remember that at any real distance, any little variation in anything (load, bullet weight/shape/depth, velocity, trigger pull, breathing, heartbeat, wind, exactly where you place your eye, trigger finger, shoulder to stock... and yes even those pesky flies!*) can easily move your POI. And for me that is all a part of the fun of it all!
And then again, could it be... primer related? Flash holes uniform? What about how the powder is sitting in the case?
It is 2012, you could always consider the Mayan Calendar; planetary alignment? neutrinos?
Let us know what you find out!
*A smaller yet similar version of "The Butterfly Effect" known to afflict rifle shooters from time to time... for no apparent reason...