In Microsoft Flight Simulator 2020, on the airbus A320NEO, why does climb take forever and then descents 1000 feet sometime during cruise?

The max altitude for the that plane is 39,100-39,800 feet.   When you get close to max ceiling things do not act consistently.   Stay around 38,000 and you will not have problems. 

The ceiling of the Airbus A320neo is between 39100 and 39800 feet.

You know what an aircraft ceiling is, right?

"With respect to aircraft performance, a ceiling is the maximum density altitude an aircraft can reach under a set of conditions, as determined by its flight envelope."

(This can be found on the internet, but take it from me, that is exactly what it is, I am an aerospace engineer who spent decades in aircraft performance).

The aircraft ceiling is not an arbitrary number, it is the result of the decisions taken during the design process and the consequence of the airplane configuration, in the present case, those would be the wing dimension (which provides less lift at a given true airspeed at higher altitude due to the reducing atmospheric density; which could be compensated by flying at a higher speed -- relying on the *equivalent* airspeed to make up for the lower pressure -- but that is limited by the ability of the engines to provide the required excess thrust to fly faster, and by the fact that the speed of sound is not much faster, and that massive drag increase occurs when inching towards that speed unless the design takes it into account), the current weight of the aircraft, and the available engine power.

As the aircraft climbs, the reduced density ALSO reduces the thrust the engine can provide, with less thrust comes the reduced ability to fly faster and to fly higher, as excess thrust is required to gain altitude. An aircraft ceiling is defined at the altitude when the aircraft climb ability is less than a few hundred feet per minute; in other words, you don't climb because the aircraft can't.

That's it.

Seems to me in this respect, Flight Simulator is giving an accurate model.

As to what a simulator might do when "flying" at cruise (what altitude?) then it could be the result of modeling the variation of the atmosphere, passing above zones of varying temperature, updraft and downdraft.

If you are not happy about how the simulator works, consider playing something else then, like Candy Crush...

Please go ask in the GAMING FORUM