Since you have checked the obvious multiple times, you need to check the unobvious (made up word here).

Check cable routing.
Make sure every ferule is seated correctly.
Check the housing is fully inserted into each cable stop.
Make sure the cable and housing are fully seated in the shifter.
Really inspect the cables and make sure it didn't fray during the installation.
Made sure the jockey pulleys were installed correctly, floater on top etc.
Check the side plates of the chain of the chain to ensure it is a 971. You never know wrong packaging.

If all those are up to snuff, then you really need to think shifters and rear derailleur or hopefully someone else has better ideas.

Good luck.