Another weird thing is how the aircraft react to thust change when A/T is off. In cruise when you do a very slight power up adjustment you can see the aircraft constantly accelerate for 10' minutes long and more. I got from 0.82 mach to 0.86 mach in more than 15 minutes with a very limited power change when I was stable at 0.82 mach.

This is really unrealistic because the air resistance is not a linear function but a square function of speed. And that means that for a slight power change you should see a slight speed change rapidly that get amortized quickly and settle in the next equilibrium state of power/drag ratio. There should't be a large change of speed at a low rate how it occurs here.