I would, without knowing you or your sighting in procedure have to agree with Dogleg. When a barrel heats usually you get vertical stringing. I believe you said it is a Model 70 lightweight, which means a pencil barrel. Given that the laws of physics are not at fault, something in the setup is.
What i would do is check the torques on the mounts, and shoot really slow groups with a cold barrel. I am always happy and excited to be at the range, so i take a 22 along and shoot my hyperness out with that. Aimed center fire shot, mag or two of 22 lr, another careful center fire shot....at least 1 minute and better yet 5 minutes between center fire shots. As a shooter, where else would i rather be than at the range. If i dont have the time to shoot in this way, i dont go at all. YMMV