Vertical stringing is often caused by forearm stock pressure. Is the barrel free floating? Many rifles these days are made with a "pressure pad" near the end of the forestock. This can be removed with a dowel wrapped in fine sandpaper. Once the barrel is floated, if there are still problems, the action can be bedded. These problems are usually fixed by floating and/or bedding. Be sure to re-seal the wood if you sand it.