Chain Jumping On Road Bike27 Jun 2013
That may be the only way to get to the bottom of your problem. I had a noise in my bottom bracket on my Bianchi that when I took it to the shop it kept checking out “OK”. I rode it on the trainer and sure enough there was the noise and they fixed it for me. The bike was new, under warranty, and the threads on the bb were not greased. Only made a noise under load, just like your chain only jumps under load.
I assume you are talking the crank here, and you have a tripple, if i am picturing things right. well, they did cables, so they had both derailers loose and likely adjusted them. are you hearing the chain rub before this happens? if you are, they may not have it adjusted quite right, so that it hits and kicks the chain down under a load as the bottom bracket flexes. it could also be a worn or damaged tooth on that middle chainring.
see what happens is the shop puts it on the stand, runs it through the shifts and it hits them all, so they say there is nothing wrong. yet under a load, something is just off enough to cause a problem. next ride, listen for chain rub, and watch how the front derailer is clearing the chain. make sure the chain has plenty of clearence and is centered in the cage (between the inner and outer plates of the front derailer). i am betting you find it is nearly rubbing the outer plate, and as you put a load on it, it hits and makes it shift. i am betting that front derailer isnt set right.