It outta shoot better than that with any ammo. Check your bases and rings. Then, run a dollar bill between the bbl and stock to make sure its not walkin on you.
Glass bedding will tune one in, but if its shooting that bad, I cant see that helping that much.
If the bbls floated and the rings and bases are tight, you may have to take it to a smith to check the bbl. Make sure its not "alligatored" from abuse and rust or simply shot out. I have seen some really nice looking guns that you would swear were like new that had bad bbls. Worst case would be order a new bbl. Action should be good. Alot of fine guns are built on 700 actions.
One other thing, look at the crown on the end of the bbl. A nick from dropping on concrete or something can make a bullet do wild things.
Andy
Edit: As I have a habit of doing, I get to thinking outloud then go back and read the question again.
If it tightens up after shooting and heating, it could well be the bbls not floated. Thats an easy fix. Run a dollar bill down it. (or a $100, it dont matter.
) If it wont easily slide from the end to the action, that would be a good place to start. I had a Ruger do that one time and thats what it was. I think what was happening was after a few shots, it would finally settle on a spot and stay and shoot good for a while.
When it comes in after a few shots, does it shoot in the same place everytime in relation to the bullseye? Or is it just a good group in different places?