Greetings,

@Colvale: Alright, so it was from the beginning a "measure-setting" problem. You should never change this after a fresh install of FSX. Always use of US-Units (Feets/LBS) is recommended. If you need a unit in another value, recalculate it with the corresponding formula. There are many good sites in the INET that made that for you  :)

@Markoz: I recommend to use my tipp in my reply #1 and you have a stable 707 also in high altitudes  ;) I guess this depends more with the AP.