Originally Posted by gugie

Looks like you're well on the way to fixing your issue, but I have an old cyclocross trick to offer.

I see you've got the right shifter going to the right side cable stop (drive side). That gives the housing a pretty tight radius bend, which results in more friction. Instead, reverse them, so right goes to the left side, and left to the right side, then cross them back over under the down tube. Even though the two cables are now rubbing, the overall effect is less cable friction.

Of course, this assumes that you have under the bottom bracket cable guides. If you don't, it's a simple matter to drill and tap an M5 hole in the bottom bracket and put a plastic under the bottom bracket cable guide in.