I had a similar problem with a Tricross, not sure if it's the same but here goes.
The front derailleur could not get the chain onto the big ring without a struggle, if I put it on a stand it seemed to shift fine, it was only when I was riding it.
I'd had the crankset replaced by my LBS a year before this and it turned out the BB was the incorrect width for the new crankset, I'd had it into three different bike shops before someone worked out what the problem was. I got it replaced and the problem went away.