I'm betting the chain has worn even more by now. How many miles have you put since you got the new cassette?

A few things came to mind:

- You might be favoring only 1 or few gears on the cassette. This practice has the opposite effect of accelerating wear on the whole drivetrain than if you're using the whole range of cassette cogs with terrifying regularity. This practice will also accelerate wear a lot faster on the chain and a worn chain would skip and jump between cogs, all sorts of stuff. If this is your case, you may be in need of replacing both cassette and chain soon.
- Chain is "flexing" a lot to the sides. A bit of sideways chain flex is needed but too much cause random jumps between cogs if chain line is at extreme positions (cross chaining) cause lethargic shifting. Cheap chains have tendency to flex a lot to the sides.
- Someone suggested bent hanger and this is definitely a possibility or you simply needed to tweak shifter indexing again.
- And finally, is this your bike mechanic?

Like WhyFi said, check cassette for looseness. Next people said, check derailleur alignment, and finally, like others have said, get a new mechanic. "Air in the line" ???

Yeah, hydraulic lines for derailleurs, it's a new thing! Much better than electronic.

Rotor has/had this, but I don't think it has really caught on.

I'm going to get an update from the bike shop today. Been out of town.

btw, I looked back at some text messages just now and was reminded that I had a new cassette and chain installed in mid-December, so that's 7 months ago, and I'm guessing 2,500 miles. Chain and cassette lived in harmony until just recently. Mechanic measured the chain when I brought it in this month and said it wasn't too stretched out. Diagnosis was derailleur problem, replace. Didn't solve it.

If the mechanic didn't check the derailleur alignment before selling you a derailleur, he should refund that $$$. Despite all those little moving parts, derailleurs rarely wear out. Even when they take a hit, they're more likely to pass that along to the hanger - and not bend. Derailleur hangers are like fuses. They fail before the frame and the derailleur to save those parts.

Got the bike back Monday, have ridden it twice, no more skipping. Don't know what to make of all this. Just glad it's working again. I probably need to invest time in understand how my bike works if I'm going to ride it this much.

aaaaand?

You also probably need to have a very comprehensive dialog with your current mechanic if whatever they most recently did fixed the problem. Not knowing what to make of all this means both you and he may be destined to repeat these shenanigans in the future.

iow, what did he do that finally fixed it? and, why didn't he do that the first time around?

They made more derailleur adjustments. They did the same previously, but didn't get it right. Problems weren't apparent until I went on a long ride w/ some hills.

Sounds like the mechanic finally checked/aligned the derailleur hanger. There's not that much to adjust on a rear derailleur - especially that affect the middle range of the cassette.