Shimano 105 rear shifting issues I'm disappointed with my new Cannondale and it's 105 group, I expected this to be a major upgrade from my previous trouble free bike with the bottom end group set. It's done about 500 miles, had new cables, a new chain and rear derailleur re-alignment and numerous adjustments over the 5 service visits between 2 dealers and it still doesn't shift smoothly every time. Over a 2 hour mixed terrain ride it will miss-shift several times.

It seems to be mainly an issue with the 3 black colored gears in the center of the cassette, regardless of small or big chain ring. Sometimes when up or down shifting the lever clicks as normal but the chain does not move to the adjacent gear, click again and it moves and stays there until the next shift. Other times I would down shift in anticipation of a hill, the chain appears to have moved, but when I stand it slips.

Both of these issues happen randomly and spoil my riding pleasure, does anyone have any thoughts on what the issue might be?
Assuming that the cabling is clean, that the derailleur is nicely indexed and well alligned, the derailleur hanger not bent, and the chain correctly moves lateraly +/- 4mm for each gear up or down, then I would have a look at the distance between the upper jockey pulley and the sprockets.

The distance should be on all sprockets less than 25 mm and more than 12 mm.
I suspect that your distance is slightly too high, meaning that the chain has too much room and can flex.

The way to adjust this is by loosening the B screw, as described on this page.

I had the same problem on a 105 derailleur on one of my bikes. B screw was loosened to the max, and I was still too far from the sprockets compared to my other bikes. I managed to file the part of the derailleur hanger touching the support of the derailleur where the B-screw is, to shorten this distance even more. Now it shifts very well.