747 autopilot can't maintain constant altitude 787 unsteerable during taxiing

Maybe this has been raised already somewhere, but I can't find it now.

Is it only my 747's that constantly ascend and descend (1000 ft up 1000 ft down) around the entered altitude on the autopilot?

If not, can anybody please tell me whether there's a way to fix this?

I can't use the 747 as well as the 787.

It is practically impossible to taxi the 787, it goes its own way.

Here too, if someone knows a workaround, I'd be most grateful...