I have an A. Homer Hilson bike from Rivendell with 3 x 7 gears.

The problem is that every once in a while my gears will skip. This happens frequently when I'm climbing but also at other random times too. The gear will skip to a lower gear, then skip right back.

Any ideas on what's causing this? I have about 2000 km on it and this problem has been with me since the start.
thanks,
Dave