Skipping on new chain, new cassette, new derailleur 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 &amp; 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?
You could try removing a link or two from your chain to reduce the amount of slack in it.  A chain that is too long may cause skipping.

(Ensure you can still reach your entire gear range with the shortened chain length)