Transmission problems with Honda Civic LX 2011

This is a total mystery thus far, and I'm making some last ditch efforts to salvage the car. About three weeks ago, the transmission just began to buck on the shifts. Doesn't seem to matter -- on the upshift, it's not a smooth transition from gear 2 to 3, and on the downshift it pushes forward like the car is either stuck in the higher gear or wants to accelerate when I'm trying to slow it down.

I've taken it to several mechanics so far and they don't seem to notice the issue themselves. But it definitely drives differently than it did in the recent past. I used to reliably speed up and slow down with no hesitation noticed in the car.

Around the same time, I noticed the temperature not hold steady. That is, the car cools down when moving on the road. Typically it should stay warm at optimal operating speed. When I pull over to stall the car, it warms up again to where it should be. This does correlate with the stickiness of the gears, but not always.

So far, I got it a tune up, changed the transmission fluid, replaced the gasket, got the tires aligned and balanced. Still no resolution.

It's been treated fairly well over the years, at 138K I would expect this car should have more life in it. But...Is this car a goner, or is there something I'm missing?