It's most likely your pitch trim, you need to adjust it as you disconnect your autopilot. I don't play FS
nearly as much as I used to, but I am very sure that this is what your problem is as I've had it with certain (not all) FS
aircraft. However, another problem I've had quite often is when the autopilot intercepts the g/s, the ALT hold will go off and the plane will go almost vertical for no apparent reason. Thats one problem I've still not been able to figure out, but your's sounds like you just need to adjust the trim.