Hi, I have one question. Why when I'am flying this 777 randomly my plane during flight dropes down and vertical speed goes to -4000 randomly. For example, I was approaching KJFK and about 20NM before landing my plane dropped down and vertical speed went to -5000. The same thing happens when I descending, and the plane won't hold the altitude when the "HOLD" buttom is active, and on MFD it lights "ALT HOLD". I have modern flight model, so everything might be okay...
Any sugestions?