747 altitude problem

[Daniel choen 23]

Het,as said above i have a problem with autopilot altitude,i did a flight from innsbruck to munich today at vatsim every thing was perfect, and the without quitting from the fs i did another flight from gatwick to manchester, here was the problem i did every thing i think exactly as the previous flight but the altitude, didnt kept on the altitude in fact when i engaged the autopilot the plane started to raise the nose up and climbing very fast, i needed to disengaged the autopilot and fly manual the others autopilot system worked well though, any suggestion what is the problem is?THANKS again, btw the product is new to me, i got it yesterday but i have a previous experience with the ngx though so i know how to fly it, and had some experience with it at my friend, he has this product for a year or so,thanks again