When this happened to me, it turned out to be the left 105 shifter ("brifter") It was about 1 y/o and Shimano's warranty is 2 yrs. Bikes Direct replaced it, but when it didn't match (I had Ice Grey, and they sent a shiny silver) they upgraded both shifters to Ultegra.

Of course, it could be a lot of other things: kink in the cable housing, defective FD, etc.