Mysterious Chain Jumping

[thepinto201]

The derailleurs are perfectly tuned, brand new chain, cassette, and chain rings. New rear wheel, still same problem.
Whenever i stand up to pedal hard up hills or even on flat trail, my bike makes this disgusting noise and the chain skips or something. Sometimes the chain even falls off from the chain slip. (like in the video)
http://www.youtube.com/watch?v=wU_fdH-AOV8
All components are properly lubed with the appropriate lubricants... WTF. All i can guess is something with the bottom bracket?
I am so thankful for any insight anyone has, two shops have already tried to fix it and failed.

[thepinto201]

chain too long?

[billy anders]

yo thepinto201 i am having same difficulties w/ 2 mongoose XR 250 mountain bikes. properly calibrated & lubed by professional bike enthusist . have contacted http://www.pacificcycle.com manufactures of mongoose bikes & warranty. they just keep blowing us off. my children can't ride across flat surfaces 40 feet without chain not climbing into rear spokes . need to accumate a class action against http://www.pacificcycle.com and Shimano...

[DaveM]

Tough one!<br />
Looks like the chain is falling off the front chain rings. True? Does the same thing happen on all of the front chainrings or only on the inner? It could be flex in the BB or crank causing the chain line to get so bad the chain slips off. But that's pretty weird.
Can you get it to slip at a slow enough speed someone can jog next to you and try to spot where the chain is jumping? That might help. You've changed all the parts that would normally cause this. So either you have a mismatch (incompatible) part or something quite odd going on. Sadly, you may just need to keep trying shops until you find someone willing to spend the time to isolate it.
@billy_anders - something on your bikes need adjustment or replacement. I'm not defending Pacific Cycle, but I don't think this is a 'class action' issue. You need a good mechanic to figure out what is wrong and if there is a warranty issue, have them take it up with Mongoose directly.