Newton Euler equations

Hi I'm using the newton euler equations to compute dynamics of a rigid body. I do the following: My system have a velocity described in spatial coordinates ie w = [ w_ang v_lin ]. So the velocities are described in a frame rotating with the body. I have a spatial inertia tensor I and use the following formula to compute the zero_acceleration force of the system Z_af = w x I w. Then I compute a spatial acceleration with a_s = I^-1 * Z_af and I convert this acceleration to a acceleration in a frame aligned with the body, but not rotating like a_s_fixed = a_s + [ 0 ( cross( w_fixed , v_lin) ) ] where w_fixed are the rotation of the body frame as seen from the fixed frame and v_lin are the linear velocity of the body. This are a standard way of converting derivatives between frames that are fixed and rotating. This works quite well, when the body only rotates about one axis, but when the body initially rotates about two or three axes the angular part of a_s_fixed does not become zero and my body starts to accelerate, which are wrong as no external force are working on it. I guess it is my conversion from rotating to fixed coordinates that are wrong, but how Thanks Niels