For a long time I have been having issues with chain skipping on my 1999 Trek 6500. I took a video to illustrate exactly what I mean by "skipping":
Over the last few months, in the course of trying to fix it, LBS has persuaded me to replace the chain, rear wheel & cassette, rear derailleur, shifters, and cables. Derailleur hanger angle has been checked, limit screws and cable position adjusted. None of these have completely fixed the issue.
As it is now, I can reliably reproduce the issue by riding over a large bump, or doing a bunnyhop to cause a physical shock to the bike, then riding for several seconds while standing up and accelerating hard. If I remove the bump from the equation and just accelerate over smooth pavement, the issue does not occur. There is generally one skip per bump - i.e. After a skip occurs, I usually must induce another bump before it will skip again.
What other candidates are there to investigate as the cause of this issue, and how would one test them?