The teeth on the derailleur pulleys should be rather flat, kind of like looking at a mountain with the top cut off. If they are pointy, that tells me that there is some signficant wear in place. A stretched chain can cause skipping i imagine. Go here and you can see how to measure chain stretch.
http://bicycletutor.com/chain-wear/ The teeth on the derailleur pulleys will wear first due to them being plastic.