Had a similar problem 2 days ago. I had my 7mm RemMag sighted in at 100 yds using Federal 175gr soft points. Went to the range to dial it in and develop my drop charts with what I want to use as my LRH round, HSM's 168gr Berger VLD load. Set the target at a ranged distance of 100yds. First shot wasn't even on the paper. Thought maybe it was due to the cold barrel. Fired several more and it was all over the map, high left , way right, way low right, etc. We're talking 8 to 10 inch spreads at 100 yds. Wasn't sure what was going on as I had it well dialed in with the other loads and really only expected the 168 VLD's to be somewhat higher only. Tried some of the 175 gr again thinking I may have gotten a bad lot of HSM ammo. They were not nearly on either. Checked for scope loosness and all the other things, ie. stock to action screws, etc. Nothing.
Packed it in and when I got home I removed the stock from the action and gave everything a close scrutiny. Couldn't see anything that looked like the culprit. Removed the scope from the action mounts, rings stayed in place on the scope and low and behold I think I found it. Seems the through bolt in the rear ring was not set in the slot of the weaver ring base but in front of it. I made the correction and feel confident I found the source of the problem. Recoil on the magnums can be pretty severe and something like this will certainly cause such issues even though the scope felt like it was secure. I'll confirm in a couple of days that it was the problem when I can get back to the range.
If necessary, take it all apart and put it back making sure you get everything the way it should be. Means having to re-sight again but at least you know what you are starting with.. Its often the small things that bite us in the butt