I can totally feel your pain. I bought a new to me rifle a few months ago. I finally got all the parts and pieces gathered up to get the scope and stuff on it and took it out to the range for a quick session. This was literally my first group out of the rifle after a three shot sight in. this is five shots into about 3/8". I was on cloud 9. It was Hornady factory ammo, and I am not an ELD-X fan so I loaded up some of my preferred bullets to try out and went back to the range two weeks later. Nothing shot even close and most were over the one inch mark so I thought I will try some more of the ELD-X ammo to get me on the bull at 200 yards and go from there. It took two rounds to get it into the bull at 200 yards and then my five shot group was the size of a baseball. If I hadn't been shooting my 6.5 x 284 as well the same day, and it shot fantastic, I would have just chalked it up to a bad day at the range, as I have had some of those.
Anyway I have not been back to see if I could figure it out as too many things keep getting in the way but hunting season is upon us here in Montana and I need to get to it. Good luck in your quest to figure out what the heck is going on and please share maybe it will translate to my issue as well.