I can see a few possibilities: your LBS isn't doing a good enough job in tuning this in, you just haven't found the right setting yet, or it's a problem with the component set. I have an allez sport with tiagra shifters, 105 RD and tiagra FD. I had problems with the RD shifting in similar circumstances. I was able to tune that relatively easy. However I had trouble with my FD and screwed with it enough that I just couldn't get it to work properly (might haver something to do with the fact that I broke off the cable-end ). I took it in for a tune up. If it's still giving me trouble I'm going to try minor adjustments on my own (only to the limit screws, not the cable fix) but if that doesn't fix it I'm going to assume it's just an issue with the tiagra FD.