Ranger63

Brifters are the original 6600 ultegra Shimano says there is no compatibility issue with 6600 brifters and 6700 derailleurs.
Derailleur is the new 6700 ultegra long cage for use with the 12-30 ultegra cassette
Cassette is the new 12-30 ultegra
What's going on here is i'm missing the shifts on the center 3 cogs of the derailleur during upshifts (only)
If I upshift past the intended cog and then downshift the shift is precise and crisp.
This is occuring with both the 50t and the 34t chainring.
The upshift is precise and crisp on the lowest 3 cogs and the largest 4 cogs.
The derailleur shifts fine if the bike is hand cranked and shifted so this id definitely a 'under power' issue.
Tried cable tension, centering derailleur, making sure the hanger is tight on the dropout, chain isn't stretched, spacer IS in place behind cassette on freehub.
Any suggestions?
Could this possibly be a worn out brifter? Given the cogs most used over the span of time I've had the bike would be those cogs the derailleur is having problems with.

Andrew R Stewart 

Try connecting and tensioning cable so that the lever's high gear position is with the chain on a couple of cogs up from the smallest cog. (So only the larger 7 cogs are in play). Did the shifting problem also move up the cassette? If not then the lever is likely OK, if so then the lever is likely the cause. Try again but now with the large cogs out of the picture and only the smaller 7 cogs in use. Again did the shifting problems move to the smaller cogs?

Also is the upper pulley bushing worn out? Andy.

AnkleWork

Check for a frayed cable.

70sSanO

I'm not sure what you mean by missing. Are you jumping a middle 3 cog or not shifting into a middle 3 cog, or are the middle 3 cogs just off and noisy? Have you tried to dial in the middle 3 cogs? If yes, how does the rest of the cassette shift?

Since the shifters are dialed in for the 4 low cogs, off for 3, and then back dialed in for the 3 high cogs, it would seem that the problem is in the shifter. But... what happens with the old cassette? If there are no issues, I would measure the spacing on the new cassette to make sure the spacing isn't off. Not saying the cassette is causing this, but before I buy a new shifter, I'd make sure I didn't get a bad one.

John

Andrew R Stewart 

AW has a good possibility. Maybe in my suggestion the Op would find it is a fraying cable (likely in the shifter pod) but AW called it out directly. So we wait and see if the OP will follow up with our answer. Andy.