Tell us more about your system, how much RAM are you carrying? I've just constructed some metaballs, both positive and negative using the same brushed silver material, then grouped them together and positioned them and finally rendered them without any problems. I'm carrying 512Mb of RAM on a 1 GHz, PIII using 64Mb GeForce nVidia card. What rendering mode are you using? If you don't think it's your system, try reinstalling Bryce.