hey friends... i have a problem when i fly in fsx.

The problem is with some of the airplanes i download.

The problem is that the airplane cant get the cruising altitude because the speed goes down and the airplane stalls.

For example.. i was flying with an airbus a319 and aprox. at 21000 feet the speed started to get down and i cant do anything about it because the throtle was full, flaps up and the airplane couldnt climb more.

This happens me with many airplanes.

Someone who can help me i´ll apreciate it. Thnks 4 the help

David