(Actually, the chain is skipping on the cassette)

I am stumped.
I have a LeMond Zurich spine bike. For the last several years I have been running 5700 3 speed shifters, front derailleur and 50-39-30 chainrings; with a 5600 short cage rear derailleur and 12-27 10 sp. cassette.
I just put on an Ultegra 12-30 10 speed cassette and a 5700 GS (longer cage) rear derailleur.
It shifts fine. However, when I am on the middle chainring, the chain skips on the cassette when I pedal hard. It is fine on the big or small chainring.
I did remember to put the spacer behind the cassette.
The chain is new, and sized properly by big-big.
The derailleur is rated for 30 tooth large rear, and there is no interference.
It was fine with the old setup 1 day ago, and, as I said, shifts fine and does not skip when on the big or small chainring. I don't think hanger is bent. Everything looks straight and lines up. I tried adjusting rear der. with barrel adjuster both ways until it wouldn't shift and it made no difference.
It is definitely skipping on the back, not the front (verified by riding partner).

Any thoughts before I bring it to the shop to ask their help would be greatly appreciated.

Never mind, it was skipping on the chainring after all.