What are the groups doing/look like? Couple pix of the groups would be very helpful.

String up and down, maybe stock under bbl is riding on the stock and bouncing as it shoots/heats up.

Stringing left to right, basically same. Bbl may be bouncing off left of right side of stock.

Bbl free floating MAY fix the above problems, and usually has for me. But some rifles like a free floated bbl with a ~1" pressure point at the fore end.

Other issues can be loose scope bases/mounts, action screws are too tight/binding the action, recoil lug not bedded properly and bolt handle hitting the stock

There is also a possibility that your shooting form is causing the grouping problem.