Weird shifting problem with RD 105 ?!?

My bike is new (~300 Km) full Shimano 105 9 speed front 53-39 and back 12-25.

I am having a problem shifting at the end of cassette (smallest cog) while I am on the big ring. When I'm on the road, shifting is sloppy and won't shift at all from 13 to 12 cogs. Sometimes I have the same problem in shifting from 15-13 cog. When it's in the stand or when I am on road very ease on pedal, shifting is perfect, works how it's supposed to, and I can't detect any problems. When I'm in the small ring, shifting is just fine. It's only the big ring where I am having trouble.

I have taken the bike to LBS to fix it after reading many posts here where I found to check the hanger alignment and I have insisted they check the hanger alignment. They correct the hanger aliment (and check other things) but the problem exists.

I visited techdocs.shimano.com before and download FD and RD docs. I tune FD and RD without problem (I have two another bikes and there is no problem in tuning FD and RD).

The problem is weird because is appear only under load on road but no in the stand or when I am on road very ease on pedal. I can shift from 13 to 12 cog but I need to tap two time shifter to make click and drop from 13 to 12 cog.

Any ideas where to look next?

Replace the 4mm housing with 5mm. Make sure the under BB guide is clean. Ensure the lower limit screw is such that the rear der easily moves up and down the cogset w/o the cable attached. BUT what you describe sounds a LOT like a slightly bent derailleur hanger.

I suspect that my rear mech may have a weak return spring. How can I check it and how to replace?

Unhook the der wire and while pedaling it, mounted in a work stand, push the der up the cogset with you thumb and release. It should easily and quickly return to the highest/smallest cog. If it doesn't and the limit screws are proper and the der hanger isn't bent, it's the derailleur.

Ok, I try it and RD easly and quickly return to smallest cog.

Then something is preventing it from easily going to the high gear/smallest cog when everything is attached and that is the cable/housing combo. Again, new inner wire, 5mm housing, clean under BB guide and it's always better to have the housing a little long rather than a little short.

I have solved the problem and thank you all to participate in solving the problem. I hope this could help you if you face the similar problem.

The housing near RD was bended immediately upon leaving the barrel adjuster. This was actually bending the housing end cap. The first image show how was before.

When I put the housing below quick release axle the housing near RD is not bended immediately upon leaving the barrel adjuster. See the second image.

Yes this is temporary solution and I need a longer housing in this case allows the housing to enter straight.

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