Actually, a weak older battery could cause this problem. But the first thing you need to do is clean all of the electrical connections between your battery, alternator and voltage regulator. Also make sure that you do this with both the positive and negitive connections. Many, many times I have seen someone take the time to clean all of their positive connections, only to have the negitive hookups be the problem. After you have cleaned everything, make sure that the drive belt is tightened to the correct tension, if it is loose, it will cause the alternator pulley to slip at idle. Once you have done both of these things, if it does not solve your problem, then start to look for a bad component. In other words, take the time to eliminate the stupid stuff before you replace something that isn't bad.