Thanks for all the input. The bike is a 1994 Specialized Hard Rock that I bought new, but then gave to a friend a couple years ago. He recently gave it back to me and this is when I noticed the chain slip. I would say the bike has ~10,000 miles on it.

I am on my way home now and will take a closer look at it. I think I had better also check the chain wear on the good MTB. No problems with that one yet, but I guess that's the point.