Might be the barrel, if you have access to someone with a borescope I would have them take a look.
To get an idea of the condition of the bore, poke a Q-tip an inch down the muzzle to reflect light and look down it with a magnifying glass with a strong light behind you.
I've got a 300 win mag that shoots like that and after I looked down the barrel I can see why.
I'm not spending another dime or investing any more time on it. It is what it is and it's "good enough" for big game, but forget about precision.
