In reply to Lynsety:
If the chain doesn't sit properly on either the cassette of the chain rings, it suggests a very worn chain. How long since you changed it ? Around 2-3000 miles causes enough wear in the rollers in each chain link to warrant changing it. If you've not changed anything since 2010 and used it a reasonable amount, then everything might be a bit worn out. Which is probably not the answer you were looking for.
Put the chain on onto the largest chain ring at the front (on the pedal axle). Take hold of the chain at the very front of the chain ring and pull it forwards. If you can pull it just clear of the teeth on the chain ring, the chain has worn too mutch (i.e stretched) and needs replacing. That should correspond to about 2 or 3 % stretch in chain length (which is what received wisdom says is quite enough for a chain).
If you put a new chain on and you have gear changing problems, the chain rings or the rear cassette may need replacing.