Ok, I'm totally humble here to answer, but since I am having and had this same issue with my mtb I'm putting my vote in for "cables" as the problem source.

The mtb was (and still is) doing the same thing. My kids somehow have the habit of throwing their scooters into the bike rack... which leads me to cursing and preying the bars out of my mtb rear wheel...

I came here and searched my problem and geeked out good ole Sheldon. For srue I thought it was the derailer. I didn't feel confident enough to adjust on my own, and E-ed one of my guy bike pals w/pics. Needing my bike for the holiday w/e camping trip, I loaded her up to the shop. The tech (and my guy pal wrote back by time I got home) both said "cable stretch"...

The tech didn't even take if off the vehicle rack. He showed me how to adjust it at the bars to tighten the cables. Sure enough it got better. Rode it on a flat at home, was ok. Then when I was climbing hard on the trail... bam, right back. Worst time to happen of course. I about bit it in the steep drop off as the bike didn't perform as expected.

So, my guy pal says that bar adjustment is for "micro" stuff. The piece towards the back end of the cable is for "macro", bigger adjustment. He said try turning it one way... if gets worse... then go the other until it quits. I'm lucky my suv bike rack makes a great work stand. For ME, I just don't feel confident enough to know when to stop turning the adjustment knob. I guess if you have turned and turned the thing... well, the cables have had all the adjusting they can take before replacement.