Bigcountry and Drilling Man bring up good points that you will want to rule out. I'd sure be suspicious of the actions screws, scope and mounts, but...

Some rifles, however, need some fore-end pressure to shoot well, especially light, whippy, tapered hunting barrels. All rifles will behave somewhat differently, even different specimens of the same make and model. You might try putting a matchbook cover under the fore-end to see if this helps stabilize it.

I had a Ruger M77 MkII .30-06 that went from bad (3" groups) to horrible (5"+ groups) when I floated the barrel. Others with this same rifle have floated the barrel and enjoyed much better accuracy.

My gunsmith and I tried a bunch of different things over a couple of years with that rifle, all to no avail. I ended up getting rid of it. Too bad, I loved that rifle, just couldn't shoot it in public. I have several other rifles that shoot very well with the right handloads so I'm over it, but it sure is a shame when your rifle turns into a puzzle you can't figure out. Hope the new owner can sort it out.