"Phantoms" in my scene using spheres/ellipsoids

Good morning,
I have a very very strange problem: I’ve created a 3D visor, that is, a program that displays shapes according to an XML file. The problem is that, as soon as I’ve started introducing spheres (and ellipsoids rendered as scaled gluSpheres), some spurious shapes appear/disappear from my scene according to the current camera orientation.
I’ve taken a couple of pictures just to be clearer:


I’ve rendered shapes using display lists, the one regarding ellipsoids/spheres is the following:


  /**
     Quadric object
   **/
  GLUquadricObj *Sphere;

  /**
     Obtain a new quadric
   **/
  Sphere = gluNewQuadric();
  gluQuadricNormals(Sphere, GLU_SMOOTH);
  gluQuadricTexture(Sphere, GL_TRUE);

  /**
     Start filling display list
   **/
  glNewList(dObj.listID, GL_COMPILE);
    /**
       Set polygon's filling
     **/
    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);

    /**
       Set ellipsoid's color
     **/
    glColor4d(eo.ellipsoid_color_R_,
	      eo.ellipsoid_color_G_,
	      eo.ellipsoid_color_B_,
	      eo.ellipsoid_color_A_);

    /**
       Save current modelview matrix
     **/
    glPushMatrix();

      /**
	 Translate to reach ellipsoid's center
       **/
      glTranslated(eo.ellipsoid_center_X_,
		   eo.ellipsoid_center_Y_,
		   eo.ellipsoid_center_Z_);

      /**
	 Rotate around the Y axis
       **/
      glRotated(eo.ellipsoid_orientation_Rho_,
		0.0, 1.0, 0.0);

      /**
	 Rotate around the X axis
       **/
      glRotated(eo.ellipsoid_orientation_Theta_,
		1.0, 0.0, 0.0);

      /**
	 Alter scale factors so as to obtain an ellipsoid from a
	 sphere
       **/
      glScaled(eo.ellipsoid_scaleFactor_X_,
	       eo.ellipsoid_scaleFactor_Y_,
	       eo.ellipsoid_scaleFactor_Z_);

      /**
	 Draw the sphere
       **/
      gluSphere(Sphere,
		1.0,
		eo.ellipsoid_nSegments_U_,
		eo.ellipsoid_nSegments_V_);

    /**
       Restore previous modelview matrix
     **/
    glPopMatrix();
  glEndList();

Does anyone have any clue why this happens?
Thanks in advance to anyone willing to help me.
Roberto