Charlie,

I've had this happen too. One thing you can try: stand over the bike, as if you are about to take off from a stop, with your take-off foot at the three o'clock position or so. Apply both brakes, so the bike won't go anywhere. Gently apply more pressure to the pedal, until it skips (if it does), then back off. Now do it again, this time watching the chain.

If the chain moves forward as the skipping occurs, it is probably not the chainring (may be the cog). If it does not move at all, it is probably the chainring.

You can repeat the test in a different chainring. And you can test in different cogs.

You can actually see the slipping as it occurs.

I once filed the teeth in a worn (middle, 36T, aluminum) chainring, and it completely fixed the problem. It takes a little testing, but it worked fine. It is best to use a round file that has a diameter close to that of the rollers on the chain. I used about fifteen light to medium-light strokes with the file. That wasn't enough (when I tested it), so I repeated the process, and it worked.

Putting a new, unworn ('unstretched') chain on the bike, along with somewhat worn chainrings or cogs, can cause this problem (skipping when force is applied). The chain climbs up and over the ramps. If you file them (the ramp sides of the (roughly u-shaped) chainring valleys, the ones that engage or pull on the chain) back into the right shape, the chain will not climb up.