Yes it is usually when shifting to the next smaller cog - somewhere in the vicinity of 5 to 4, 4 to 3, or 3 to 2. I hit the small brifter lever, and I hear "click click click click click" for as much as 10-15 seconds, until the chain finally drops down to the smaller cog.

I've been meaning to let the LBS look at it, just haven't had time. The RD cable housing appears to be in good order. The bike is a 2011 model but I bought it new 6 months ago. You think the cable might just need to be lubricated?