I have nothing but very good accuracy with the 350 gr. FPB bullet. Just recently I have been able to get my hands on the 300 gr. bullet and will have it at the range as soon as possible.
On accuracy, I have matched your results at the 100 yd. and the 50 yd. I am having a problem going over your problem, what with all the variables. There must be something you changed, that is giving you this new situation. Try going over it in you mind and maybe it will come to you. I can't see this just happening out of the clear blue!