I have 105s on my Giant and have had the problem with it coming off the front. I've done 2 things that seem to have helped. 1 is to shift way up in the back before shifting up in the front.

The other is to continue pedaling until I'm positive it's off. Shifting up in the front is a little noisy sometimes so I was assuming it was slipping off. But stopping pedaling simply didn't give it the time to finish the shift so it would fall since I had stopped in mid-shift. So try pedaling a crank or 2 more and see if the chain picks up.

HTH