(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.

So a new cassette and rear der with old chain?

If so first thought is a chain/cog wear miss match skip. With the longer cage but the same length chain the cage will rotate so the guide pulley is further from the cog's undersides when in the middle ring, compared to the granny ring. And when in the large ring the entire der gets pulled forward. So in both large and granny the chain can wrap further around the cogs. Just speculation. But a new chain is an easy try and what most will say should have been replaced when the cassette was. Andy

Are you certain it's the cassette slipping rather than the middle ring? I had to replace my large chainring recently for exactly your situation. I could never see the chain slipping on the big ring but the problem went away when I replaced the chainring.

gregf83's suggestion is quite possible, we find this a few times a year at work.

But it's my naïve nature to initially believe the person who has the bike in front of them. Andy

Thanks for your replies,
I guess my edit was missed.
I am even more abashed than if I had had to take the bike to the shop.

It was skipping on the chainring, not the cassette.
I had put on a new chain.
When I put the old chain back on it worked fine.
I have ordered new chainrings.

I must say, this is the first time I have ever had to replace a chainring, and I don't let my chains get too long before replacing them, so I was blindsided by this and the fact that my friend said he saw the chain skip on the cassette.

Oh well, don't believe what you hear, and only half of what you see!

How do you measure them? Chain checker? :-( Ruler? :-)

Sort of glad to hear it was the chainring, wracking my brain trying to figure out how it could have been the cassette.

I use a chain checker, and change at .75. As I said, I've never had a problem with chainrings. I was still running the original cassette (13 years, 2-3K miles/year) with no problems.

Expect some to now lambast commercial chain checkers. One aspect of chain wear little talked about is the lateral flex and how that degrades shifting over time. Andy