I'm betting that seely's right. Since this is an older bike, you might want to take a shot at fixing it yourself. This home remedy isn't as precise as using an alighment gauge, but I've had good luck with it.

Shift your bike into a gear that makes your derailleur hang straight down. Now stand your bike up against a wall so that it's perfectly vertical. Now look at your derailleur arm from the back of the bike. It should be exactly parallel with the plane of your main frame tubes. If it's bent inwardly a bit, that's your problem. Clamp a big crescent wrench onto your derailleur and gently bend the derailleur hanger back until it's straight. It doesn't take a lot of force, that's why they get bent so frequently. Resist the temptation to remove your derailleur so that you can clamp directly onto the derailleur hanger. If you do that, you'll probably distort the shape of the derailleur mounting hole.