New bike: Ultegra 6700 components - rear derailleur (GS) with 11-28 cassette.

On the stand there is no problem, it shifts perfectly. While riding, it's a different story. Although shifting through the rest of the gears is normal, the chain feels and sounds like it wants to shift from 19 to 17 with normal pedal pressure and actually does shift with the added pressure of standing. What is more confusing to me is that, with a 1/2 turn adjustment on the fly, the problem seems to go away or at least get better, BUT the next ride... it's back again. Is there something that could be loose? ?? I can't understand what is going on. Any help to rid me of the horrible crunching sound of unexpected shifts is greatly appreciated.