Bicycle Mechanics - HELP! Crappy shimano 105 shifting!!!

Hi all;

My ****ing 105, brand new on my opera bernini has been giving me the ****s since I got it, about 2 1/2 months ago.
I have a (stock, unfortunately) 12-27 cassette, and I really don't use the 27 or 24. I'm mainly interested in grinding and sprinting.

I'll describe the behavior:

Limit screws are set correctly. (Really, I have to affirm this!)
Cable tension is set so that in cog 5 of 10, when in little ring, the top der. pulley is aligned with the bottom of the cog. (As it should be!)
Result: Great shifting across the range, a little hesitant to hit the 27 when in the big ring. Whilst sprinting, shifts nicely and quickly down the stack up until the 14t. When I try and engage the 13t, it hesitates, sometimes taking 2-7 seconds to engage, even if I vary my power input, same result. Same thing but worse when trying to hit my 12t.

I tried re-indexing the gears, exactly the same. I tried lessening the cable tension so I'd have quick engagement going down the stack. Same behavior in 13 and 12t. GRRRRR!!!!. So I adjusted the limit screw, so it should be throwing the chain into my dropout instead of landing on the 12t. Nope. Still doesn't want to grab the 13.
I've played with the B tension, adjusting it up, from it's relatively lax initial setting. Nada.
I tried indexing it to be optimized in 5 of 10 cogs whilst in the big ring up front. No cigar.

I'm a trainee cert III bike mechanic, and all the guys at work say "is the derailer hanger straight, hows the cable tension, it must be your limit stops" but these things have been checked over and over and over.

It works fine in the stand with no load. I'm getting ready to blow this 105 up and get campy like the guys at work told me too in the first place. For the record, my old Tiagra (2004) gave me full range in the bottom and top chainring, and shifted under full load in any combination. My 105 just won't perform, it's supposed to be an entry level race group...whats the point in trying to race when you have to wait 7 seconds to get in your top end sprint gear?

Things to check that might be overlooked.

Bolt or bolts holding the replaceable hanger tight?

Cassette tight with 10spd spacer behind the cassette if we are talking 10spd.

Sometimes you might have to back the b-tension off all the way to get it to drop down fast enough to the small cog.

Skewer tight?

Put slack in cable and pull housing sections back and forth to confirm they are sliding smooth and free?

Last one I can think of is oil the bottom bracket cable guide, seems silly but that thing will mess with you.

Non replaceable rear der hanger (Opera is owned by pina and produces some of their old models-my frame basically a 2004 prince with different tubing i think). Will definitely double check the tightness of the cassette, and my cassette body wont take a spacer. Skewer is tight. Shall check and report back.

Thanks!

Guessing, but it sounds like the problem is related to the spring in the RD not pulling it over strongly enough. That could either be from friction somewhere in the cable/housing/guides/brifter or a problem in the RD itself. You might try taking off the cable and chain and see if the RD moves smoothly and with good force out to the outer positions.

On older stuff I have had success with putting a thin washer under the RD (where it threads into the hanger) to move it out a mm or two. I think the return spring gets weakest at the outer range of movement so that moves it more into the middle of the range where the spring is stronger. But new 105 parts shouldn't need jerry rigging like that. 105 should shift very well. There is something specific going on, it's not just that the "parts are cheap" etc.

Make sure the loop of housing for the rear derailler is not too short causing binding of the cable. I'll second the comment to check the bottom backet cable guide too for proper cable routing. Worst case, replace the cable and housing entirely.

This sounds mickey mouse, but it's tripped me up more than once: is the wheel totally aligned in the drops?

It really sounds like the cable's hanging up somewhere - you're able to overcome it fine when downshifting, but the derailleur spring is not doing it when it's close to the end of its travel.

Actually +1 on joejack951's thought.
The rear loop of housing bends more and more as you go to lower gears. These can create a lot of friction if they're just a little too short. Check if the housing does not go perfectly straight into the cable stop on the chain stay or into the back of the derailleur when it is the smaller cogs. A small kink there could create enough drap to stop the RD from popping into place immediately.

The cable was fine. I backed the b tension all the way off, reset limit screws again, reset cable tension again, indexed the gears again, lubed the cable guide under the bb, and told the 105 that Athena would soon take it's place if it didn't smarten up.
The result?
I snapped my 105 chain today sprinting, but the shifting worked!
Put a 6700 chain on it, and now the shifting is even better!

See, that little talk did a world of good!

I always like to take the cable OFF the der and study it. If the problem persists, then stop trying to fix the cable/housing/shifter, if its ok, then stop messing with the der.