I've got a 2-year old bike with Shimano Ultegra components. The bike has about 8100 miles on it.

About 3 months ago, the chain started skipping between gears, sometimes three gears in the reverse derailleur at once. My chain keeps slipping off of the chainrings.

Since then, I have had a complete overhaul of these components, and my LBS has been unable to help completely cure the unusual shifting. I've also replaced the rear cassette, the chain, and the middle chainring.

The LBS guy said there may be other things to try, including:
[I] replace the derailleur cables.
[2] convert the triple to a double

I'm at a loss. Any ideas on what may be causing this? I'm about to kill myelf on this thing. I can't get out of the saddle without thinking this is the time I'll be slamming into the top tube in a way nobody wants to think about.

Thanks in advance for any advice.