Throttle and altitude problem in pixhawk2

There is no problem in stabilize and manual mode, but when I use auto mode the point that I determine before, the uav track the point but the altitude that I determine isn’t tracked. For example; I determine the altitude as 150m, the uav track the higher or lower altitude. It lost altitude between turn of the points. Also when the uav track the route, the engine is unsteady at throttle.
Thanks in advance

Which FW ?
Copter or Plane ? (it does not help that you call it UAV)
Electrical or Petrol ?
Parameter file ?
flight logs ?

typs talon

motors electrical 2820

Which FW version ?
Please provide your parameter file.
Please provide flight logs, either dataflash .log or telemetry .tlog files.