Bicycle Mechanics - need help with an adjustment please

when I shift down into my next to largest cog, my chain "hops" several times before finally settling down into the teeth in the cassette. As best I can tell, it only does this hopping on this cog.

how do I fix this? I'm running a shimano tiagra group with dura ace cables. the cassette is an 11-28 cassette. thanks in advance for any suggestion.

hanger alignment, more cable tension, worn cassette/chain. dirty cables usually manifest itself when going to a smaller cog or the relaxed position of the RD.

First try adjusting the barrel on your rear derailler, which will change the inner wire tension. (A little turn goes a long way.)

See the "Checking the Rear Indexing Adjustment" (http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur) section at Park Tool for the how-to if you're unfamiliar with this adjustment. (You'll need to scroll down a ways to see that section.)

brand new cassette, brand new chain. I'll try the barrell adjustments.... I just had the entire drive train rebuilt, so I'm hoping it's a simple adjustment. thanks, I'll give that a shot

When erratic shifting occurs only on a single sprocket it's usually difficult to resolve via the adjusting barrel, since that affects all sprockets equally. The erratic shifting you're seeing could be the result of unequal spacing of the one sprocket to it's neighbors, of if it's a loose sprocket cassette, one sprocket turned backward. These issues are rare but of unheard of, and I only suggest them since the cassette is new.

Most likely it's a hanging cable as mentioned by reptilezs. there's a definitive test to verify if this is the case which you should do before anything else.

Shift to the one sprocket larger than the problem shift, then do the problem shift, and stop pedaling if the shift delays or hangs to keep it there. Pluck the bare wire away from the downtube hard like a guitar string, then resume pedaling. Plucking this way pulls down the wire against friction, so if it shifts immediately after plucking then it's dirty, sticky, or worn cables. Repeat the test a few times so verify if it works fairly consistently to confirm the diagnosis.

The other cable related issue could be a fraying inner wire up near the levers, The frayed area will hang where it enters the housing, causing the RD to always stick on the same upshift. This often happens just before the wire snaps, and can therefore be an early warning.

The erratic shifting you're seeing could be the result of unequal spacing of the one sprocket to it's neighbors, of if it's a loose sprocket cassette, one sprocket turned backward. These issues are rare but of unheard of, and I only suggest them since the cassette is new.

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had this happen with a new sram cassette. mid to high level one with an alloy carrier. the first 7 cogs shifted like crap and the biggest 3 were perfect. the alloy carrier was not machined square so the loose cogs were misaligned and off axis with the freehub body/axle. drove me nuts till i realized what was going on

brand new cassette, brand new chain. I'll try the barrel adjustments.... I just had the entire drive train rebuilt, so I'm hoping it's a simple adjustment. thanks, I'll give that a shot

As noted could be a lot of things and not likely barrel adjustment. Best to return it to whoever did the drive train rebuild.

....of if it's a loose sprocket cassette, one sprocket turned backward. These issues are rare but of unheard of, and I only suggest them since the cassette is new. .....

Since the wide spline is offset, how do you turn a sprocket backward?

My bike did the same t hing. Took the bike on a trip and was jumping in second to lowest gear. I put it into the lowest gear and looked down to see if derailer was aligned with lowest gear. It was not, so a couple turns of the barrel screw on the derailer and it fixed it. this was an index shifter, so it was critical for alignment of derailer in each gear.