Chain skipping (jumping between rear gears) is damn annoying, especially when it happens as you are riding out of saddle. There are a number of reasons why this may happen: badly adjusted derailleur, worn cassette, worn chain. Last week, my bike started to suffer from chain skip all of a sudden. The cassette did not seem worn, nor did the derailleur look misaligned. So I decided to just give the chain a good lube and see what would happen.
The problem went away!
I cannot really figure out why lubricating the chain would fix the problem. There isn't really any literature on the internet I could find on this topic either.
Anyhow, next time if you can't figure out why your chain is skipping, get the lube out and give it a go. It may solve your problem. Who knows...