Have you had the hangar checked with an alignment tool?

 

^^I would check this first. Then I would ask when you last changed the cable and housing. Based on the mileage, they're due, in my opinion; however, I am most interested in whether or not you have low-sided on the derailleur side recently. I drove myself crazy chasing this same problem on my last bike. Went through everything with a fine-toothed comb to no avail. Noticed a fairly significant scratch on the parallelogram, so I threw on an NX derailleur I had laying around, and everything shifted perfectly- up and down the cog; under load- everything. That low-side tweaked the mech just enough to cause problems in the middle gears and produce the exact symptoms you describe.

Good luck.

 

Yes, I have. Measures straight now, but that's an interesting question... I think the shifting got worse after I bashed the derailleur hard not long after I fitted the 12x chain-set. The mech has been replaced since then... should I consider a new hanger even if it seems to be straight?

 

These are prone to bad shifting if B screw isn't perfect.. My case is that with suspension at no sag and right on the mark on the sram tool, I get perfect shifting, but its a hair chunky between gears, just a little rough shifting. If I raise the B gap a hair it shifts like butter, but then becomes lazy going into the smallest 10t cog. There is a happy medium and its not much of a window, the sweet spot is only a 1/4 turn from issues.
[Yours] sounds like a combination B screw and barrel adjuster

 

Subscribed, as mine has started this also after last weekend's race. I did slam mine back on one stage where I didn't have any chain tension.

 

Just thought I would post here for anyone reading all this in the future: I did replace the cable with a good quality one, and the shifting was a lot better. I couldn't see anything wrong with the old cable...

 

I'm a bit late to the chat but maybe this helps someone. I had the same problem with my Sram GX Eagle 520%. Also alligned my hanger and adjusted the B screw but still not perfect. Turns out my chain was a bit too short by two linkages. With a new longer chain, all my problems were solved.

 

Don't rule this out:

 

Worn chain, sticky shifter cable/housing, bent hanger, loose cable... It's something. Last time a chain ran across the rings the cable was way loose. Before that it was a worn chain on a different bike. Pulleys can get sticky and cause issues as well, but if they spin by hand they are ok 😁