It's possible you just have a bit of dirt, lint, or some other contaminant underneath a few of the keys. Try blowing them out with compressed air before you look into replacing keyboards or other components. If the individual keys can be removed on your particular model, carefully pry them up and wipe down the contacts with isopropyl alcohol. Allow them to dry before you reassemble. If neither of those things helps, look into some of the culprits that James mentioned.