Having trouble with what seems to be a skipping chain when in the 34 front and both the 12 and 13 in the back. This is my first non-campy build and it only happens in the 34 and not the 50. The skipping seems to stem mostly from the 34 ring like it may be worn.

Components:

Ridley Excalibur frame
105 shifters
dura ace fd
force compact crank
105 rd
12-25 dura ace cassette (i think)

What you are doing is "cross chaning", that is using the small chainring with the smallest cogs. It's hard on the chain and there is no reason to do so. The 50T chainring with a 19T cog will give the same gear as a 34x13.

Since the chain is angled so severly in those combinations, I expect the chain is rubbing on and catching on the ramps and other shifting enhansements on the 50T chainring and that is what you are experiencing.

I recently couldn't figure out why my chain was skipping around.. I had a feeling it was the smallest cog I got off with a hammer and punch, but it ended up the RD had a little play in it. Tighted everything up as much as possible and it works great.

I stretched out a chain, 3 years freq. riding, and it would do the same thing. Replacing worn parts is a good idea.