I've read through all the chain skip threads that have gone before, and I still can't figure out what's causing mine--or more importantly, how to stop it from happening.

I had been experiencing some chain-skipping over the rear cogs when I would crank down hard in either of the two smaller cogs, most notably in wet weather. Cleaning & lubing didn't help much, and the problem worsened over time.

Eventually did a full overhaul, as stuff was getting worn out anyway. Other problems made themselves apparent and I ended up with new bb, chainrings & rear der. as well as the chain & cassette.

So, now I'm riding on new chainrings, new chain, new cassette, new rear derailleur--and the first rainy day I ride in, the chain skips a tooth when I'm pushing hard in top gear (big in front small in back). Once again, cleaning & lubing doesn't solve the problem.

What's left to fix? I guess an easy solution would be to stop pushing big gears, but in general I find it's easier to adjust a machine than it is to adjust my habits...

Any guesses what might be causing this chain slip now that all of the usual suspects have been eliminated?

The pawls in the freehub are slipping and may be worn out or sticking.
That would have been one of the first things to look at.
You may be able to soak it in lube and loosen up any old dried up grease that may be the cause.

Enjoy

If the smallest cassette cog is an 11 it needs to have the special lockring for a cassette starting with an 11. This should have come with the new cassette.
Another possibility is too much shift cable tension.

Gotta bump this one, because the chain-skip problem is back!

Thought Powers2b's freehub pawl diagnosis was spot-on, as I actually started to get some noise from back there while coasting. Took the thing apart and found I had a cracked freehub body (!).

Well, now that's replaced with a new freehub, and everything was running just great--until the rains came. Now the chain's started slipping again if I'm starting up or coming upon an incline in either of the two smallest cogs (it's a 13-25 9-speed cassette--Campy). This happens whether I'm in the big ring or the middle ring up front.

Did I mention this only seems to happen when it's rainy? What's going on here? I clean & lube regularly, weekly in the rainy season ("ATB" II lube).

Help?

Have you check your axle ? It might be crack (a hairline crack ) which will open up under stress such as a incline and closes up when the stress is gone. The repair is to replace the axle.

Since you say it only happens when it's rainy, I'm wondering if water is washing some grit in and stiffening the chain a bit. Next time it skips, stop and back pedal while holding some pressure off the RD idler cage so the lower loop is slightly slack. Look for any links that don't immediately open up straight as the chain leaves the lower pulley. If you see any, there's your problem, and the solution is to change your chain care method to something that works better in the rain.

Rain might also somehow affect your cables causing poor trim in the higher gears. After upshifting to smaller sprockets, try plucking the gear wire away from the down tube like a guitar string to ensure that the cable pulls all the way down. If the plucking works to prevent or reduce your problem, it's either in the cables, which need a lighter lube, or in the ergolever which is resisting the RD spring and not returning to the correct location.