I have a 1997 Gary Fisher Tassajara that got a complete overhaul and tune-up at a bike shop a year ago. (to the tune of $180, but some of that was new tires- I'm the original owner) I ride between 4 and 8 miles on logging roads and easy dirt trails several times a week, and didn't have this issue until late Spring. The slipping started infrequently, but has gotten much worse in the past two months. It doesn't slip in the highest gear, and I can sometimes find a lower one that will work for me- but other times I end up having to put the chain back on mid-ride. I'm a little worried about the chain or gears actually breaking while I'm out in the wilderness alone, so have limited my backwoods trail-taking until I get this issue taken care of.