The shifter should have a cable adjuster on it, a round piece you can turn to loosen or tighten the cable. Now picture the pulleys on the derailer as they sit underneath the rear cassette cogs. The LBS is setting it up so each shift places those pulleys directly in line with that particular cog. If the cable loosens over time, those pulleys are going to get off-center towards the next harder gear..you'll notice this as the shift to harder gears coming real fast/easy. Conversely, when trying to shift to a lower gear - which is the next bigger cog - the pulleys will be a little too far away from it, which will create the effect of the chain trying to hop up to that cog, then dropping back down to a harder gear again.

So whatcha do is get the bike supported so you can fiddle with that adjustment, and tighten the cable until the pulleys are lined up under the cogs again. Good luck!