Today on my ride, shifting down from the largest chainring on the front derailler was erratic. Most of the time it was fine. But sometimes the derailler was slow to respond (did not work for several tries and then when tried again a few moments later would downshift) or did not respond at all (as in push on left brifter lever to downshift and nothing happens for several minutes- as in an entire hill). Any ideas what might be going on? Is my bike possessed by the devil? It was fine for the last part of the ride. Help.

FYI- the bike was purchased new in March, has only about 1000 miles on it, and was tuned up once at a bike shop (not where purchased) a few months ago. I bought the bike in Michigan but now live in California so I can't take it back to the original shop. Oh and it's a triple.