Originally Posted by
dddd
One possibility is that the chain was routed incorrectly thru the rear derailer cage. I've seen this! There's a bent tab on the inner cage plate which prevents a slackened chain from bowing out and jamming between the pullies, and the chain must not be on the wrong side of this tab. The chain must make a straight line between the two pullies and shouldn't be contacting the tab.
The only other cause might be a stiff-turning pully. Any friction in either pully's rotation reduces the tension in the chain between the pullies and the cassette, and so makes for poor shifting with tendency for chain to slip.
Any friction in the top pully also tends to rotate the cage toward further overall chain slackness.
One more possibility, with these newest chains and derailers, the chains are directional (asymmetric) and the pullies are directional as well. Your bike's assembler should have known!
Cheap bikes sometimes have poorly-stamped chainrings that have a rumble you can feel, but not on a good bike like a Roubaix!
Please don't take offense, but you're new here so take this advice in the positive way I intend it.
It helps to read the OP before throwing stuff against the wall to see what sticks. In this case it's a
NEW, Dura-Ace equipped bike, so most of your comments don't apply, and if the mechanic missed a chain dragging on an RD cage tab (which I'm not sure Dura-Ave cages even have) then he's even more hopeless than we're allowing for.