Aircraft not maintaining speed in managed mode nor maintaining assigned altitudes

[aingramncl]

I do not know if I am doing something wrong here but always happens to me in the 1000 series A350 no issue in the 900 so im thinking might be something i have done wrong here, but everytime I fly the A350-1000 the aircraft seems to just never want to maintain a safe speed at cruise and sometimes disenages the autopilot due to inidicated airspeed being almost at stall speed, max speed i get is mach .57 at FL330, any help what I am doing wrong is much appreciated. apologies if this has already been answered.

see attached screenshot of my altitude at FL330 and mach .57 and the optimum currently FL400