Probably an sudden change in FS weather, turbulense or change of wind direction or speed. THis happen when you cross different flightlevels because the weather in FS sets the wind in separete altitudes.
at FL120-FL160 zero wind but at FL170 wind 245/34, then you get an sudden change in airspeed. The same goes with turbulens.
You can use the FSUIPC registered versio to smooth these effects or you can also turn of the turbulence effect in FS.
I'v stalled the B763ER when trying to climb Crz levels because of this to,
you get an sudden tailvind of to many knots and the plane losses to much airspeed and you go into a sudden stall, or the aircraft can get an headwind and you bleed of the airspeed more slowly.. The stall will lead to a sudden drop of altitude, at en excessive rate witch add's a lot of G-fore almost like an rollercoaster droppen staraight down.
I hope all I said is correct otherwise please correckt me:8