Come to think of it - and I can't believe I didn't think of it before - but I kind of had this issue a couple months ago. Sometimes when I would shift from the big ring to the small ring while shifting the rear mech too the chain in the back wouldn't catch the cog - it would just spin. I had to backpedal for a second to get it to catch. After it happening 2 times in a month I did a "from scratch" adjustment of both derailleurs - I re-did all of the adjustments as if it was a new build - and it's been perfect ever since. I think it had something to do with all the cassettes I have been switching around. When I built the bike I had a 11-26 SRAM and now I was on a 12-25 Shimano. Also, I was on alloy rings and was now on Fibre-Lyte rings. Just lots of changes in the drivetrain.
So maybe you need to look at all of the things to adjust on the RD and maybe a different chain?
Just a thought.
Have you ever wondered if there was more to life, other than being really, really, ridiculously good looking?