Fully disassemble, clean and properly reassemble the rifle. I don't know how many rifles I have fixed by doing that. Old and inaccurate rifles are almost always dirty, but often times I will find loose or missing hardware as well. Then there are the extremes where a guy in the past thought he could make it "better" and he buggered up the inside of the stock so that the action could never bed properly. Get a torque wrench and look up the specs for the mounting hardware. Also, make sure you reassemble it so that action to wood fit is correct. It could be a number of issues adding up.