Bicycle Mechanics - skipping a bit

so i have a new giant ocr elite with full ultegra. when im in the middle gear and third smallest cog, the chain has a tendency to skip when im pedaling hard. this is the only instance that i can make it happen. doesnt happen during a shift or in any other gear/cog combination. the bike has less than 100 miles on it and the rear derailleur seems to line up well. it shifts fine, but i usually find myself in this gear often and its very annoying/dangerous. lbs couldnt duplicate the problem on the stand or on the street. any ideas? i dont thinks its a sticky link because wouldnt it happen in more than just this gear, right? and i feel like the chain and cog are too new to be worn down or stretched out. thanks in advance.

Skips which way - up or down? I'll assume that it skips to the second cog. If that's the case, try loosening the cable tension very slightly with the barrel adjuster. Chances are you are flexing the frame when pushing hard, causing the cable to tighten - and shift down.

i really cant tell if its going up or down.... i feel like its wandering, but it always ends up in the same cog. doesnt ghost shift or anything, maybe it just catches the side of another cog next to it or something. thankss for the advice, im going to try and fix it using trial by error.