Ok..I did some further tests and found out that the gear is causing the problem. I do however not know if its a software bug, misconfiguration of the hardware or a normal behaviour of the 737.
Gear Up = always 230kn limitation
Gear Down = 265 kn limitation
Gear in Locked "middle" Position = Desired speed
Is it normal that the plane is limited when the gear is in the full up position?
Thanks!