Could the rear of the bike gotten bumped within the past decade? If so, then there is a roughly 75% chance that this is the source of the problem. The rear derailleur hanger can get bent, causing the rear derailleur not to be parallel with the cogs, so your shifts will be off. Check this. Take it to a professional mechanic so that this can be checked. There is no feasible way for you to do this yourself, so you'll need the tools and the skills of the volunteers at your local bike co-op (if one exists near you) or the keen eye of a professional mechanic.