Ok, first off I know not to ride in small/small combinations but....

sometimes I do briefly.

I am running a 105 double front der and a 12-27 rear cluster.

When I am in the big/big combination or say anywhere between 50/12-13 combo (and sometimes 50/14) and then switch to the small ring up front while I'm still in the small cog in back the chain doesn't quite shift onto the small ring up front cleanly... sometimes it slips almost between the chainrings.... it will 'jump' onto the front ring if I shift the rear der to the 14 or higher...

so,

my chainrings appear fine. chain is relatively new.
my chain is the correct length according to most sources.
my height of my front der might be a mm or so high... i could try lowering... it is maybe 3 mm off the big ring so it could be that....
I have limit screws adjusted to where the front der has just enough clearance for the small the 34/27 combo and the 50/12 combo.
It appears I have the der centered as well (angle)

I have no other shifting problems on the bike.

thoughts? ideas?

Thanks!