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

Sounds like the rear derailleur is slightly out of adjustment, and you're flexing the frame enough to cause it to autoshift.

Do you have friction shifters? If so you probably need to tighten down the right shifter.

The most common cause is wear of the chain or cogs. Google measure chain wear, the sheldonbrown.com result is one of the best. Note that chain and cog wear together, so often replacing one means replacing the other.

He said he's had the problem since the start, so I'm suspicious of wear being the cause (unless the drivetrain was already worn when he got it.)

The fact that it upshifts then returns points to, as Scott said, a trim or deflection issue. Odds are the trim is adjusted at the outer limit of what it takes to stay on a sprocket, and deflection under load is enough to have it hop down. Also, a poorly aligned hanger (idler cage not vertical) can make the system more sensitive to this kind of stuff.

Adjust the trim inboard (the barrel comes out) as far as possible while still retaining crisp upshifting -- probably 1/2 turn or so. If this solves it, you're good to go, if not you can then explore more expensive possibilities.

Wow, thanks folks! I was hoping there was a solution other than replacing chainring and sprockets etc. I experimented a bit on my ride this evening and I do think it's a combination of minute slippage of the right shifter (will tighten that up a bit) and some misalignment. Now I know what to do! Thanks for all your help, I really appreciate it.
Dave

I'm having the same issue with my fairlyy new Giant Escapee 1. Thanks for the responses. I'll try trimming it a little first.