Bicycle Mechanics - DA 9 spd does not shift well in the middle cogs?

It shifts really well in the two largest and smallest cogs. I am running 13,14,15,16,17,18,19,21,23, and a 39 53 . Any ideas? My hanger is dead on and was just checked.

what chain are you using and is how many miles does the chain and cassette have?

sarm, brand new. the old DA chain was doing the same thing that's why I changed it. The cassette have 7000-8000 miles, maybe? It did it on another DA cassette also which is much newer.

My first instinct when there's sloppy shifting in the middle of the cassette is to look for sticky cable movement. Dirt, rust, fraying of the wire at the lever or thickened, dry grease in the housings can make the cable return slow or incomplete on upshifts, making accurate trim impossible.

My definitive test is to do a down shift, followed by a 1 gear upshift, and expect poor response. Then firmly pluck the bare wire away from the downtube like a guitar string. The extra tension of the pluck pulls down any hung up wire, and if that corrects the trim you know it's cable related and can proceed to fieldstrip the cable and inspect, relube or replace as necessary.

BTW- if this was a fairly rapid onset, take the extra time to remove the lever and inspect the wire at the lever. Very often a fraying wire will cause sloppy shifting before it fails completely. I was reminded of this on my commuter, which started shifting a bit worse, and I decided to put off checking things until I had a bit more time, but the bike had other ideas and I rode home in high gear Tuesday when the cable broke.

My first instinct when there's sloppy shifting in the middle of the cassette is to look for sticky cable movement. Dirt, rust, fraying of the wire at the lever or thickened, dry grease in the housings can make the cable return slow or incomplete on upshifts, making accurate trim impossible.

My definitive test is to do a down shift, followed by a 1 gear upshift, and expect poor response. Then firmly pluck the bare wire away from the downtube like a guitar string. The extra tension of the pluck pulls down any hung up wire, and if that corrects the trim you know it's cable related and can proceed to fieldstrip the cable and inspect, relube or replace as necessary.

BTW- if this was a fairly rapid onset, take the extra time to remove the lever and inspect the wire at the lever. Very often a fraying wire will cause sloppy shifting before it fails completely. I was reminded of this on my commuter, which started shifting a bit worse, and I decided to put off checking things until I had a bit more time, but the bike had other ideas and I rode home in high gear Tuesday when the cable broke.

Great post.

OP, you may also check that the movement of the rear derailleur is smooth without the cable attached, just to rule anything out in that department.

Derailleur is clean. Normally it should shift smoothly in all the cogs not must in the middle, right? You guys really think its the cable? Tell me about the shifter.

Most often the housing of the RD loop. I periodically replace this section of housing to restore shifting. Since the end of cable get frayed in use and would be difficult to thread through the new housing, I replace the cable too. Exercise care in squaring up the housing ends.

With the cable detached, does the derailleur move freely? That is, are its pivots lubricated and free and the spring tension OK?

Could be a dirty shifter, could be dirty and corroded cables and housings. I would hose out the shifters with WD40 and probably replace all cables and housings. I agree with Looigi that the short housing at the rear derailleur is often a problem because it is subjected to more dirt and general crud and because the short radius curvature has more internal friction.
Lots of good advice from others, above too.

If all else fails check the Der. hanger alignment.

At this point, Its just easier just to ditch all the cables and housing and replace with new. I have never done this, but what the heck do you hose down with the WD40. I have a pair of newer shifters but the left doesn't retract to the little ring. This is a bike I do not often ride. People had told me to shoot down with wd40 to lube and it will come back. I haven't done it yet. I suppose hang the bike up on its rear wheel and shoot as much as I can through the rear of the shifter?

Before you take things apart, and spend dough replacing parts, do the "guitar string" test I described earlier. If plucking after shifting, doesn't correct trim odds are it isn't tha cable or a sticky lever after all.

Its time to change cables. they are at least 7 -8 years old. I will try your method tomorrow before I rip out anything. Thanks for the tip.
Jeff