Not always, but when you string vertically it is the gun or loads, generally. When I am string vertically I try to add powder if I can, if not then I take some out to see if it solves my problem. Horizontal stringing is sign of trigger pull or shooting technique (or a lot of wind). If you do not hand load or different amounts of powder do not work, then I would look at the fore end for trouble, as Tim has explained. 
Actually, I would disagree with this statement. As I was taught in the USMC, vertical stringing is usually caused by breathing. In my personal shooting experience, if you you are stringing the shots in a consistent pattern and in a specific line (either vertically or horizontally or diagonally) and are breathing properly, then it is usually a mechanical problem...i.e., heating up the barrel and causing undue pressure points or an inconsistency in the methodology of shooting. Believe it or not, a lot of loaded rounds would be saved if the shooter would take the time to time to dry-fire the weapon and learn how not to exert undue or inconsistent pressure at the moment the trigger is pulled.
If the grouping is erratic, it is then I would start looking at velocity variations, load consistency, barrel problems, etc.
Just remember that it sometimes isn't the "dope" on the weapon but the "dope" behind the weapon that causes a majority of problems.
