I don't have the solution to the streaks ... but I have found that if you restrict the number of lights in your scene it helps a lot.  I use the IBL overhead raytrace shadows on NO AO, one Key light to the front and slightly to the left with  Raytrace shadows set to 15 and Ambient Occlution on and sometimes when needed a fill light at low intensity to the right with or without shadows and no AO. According to the balance of intensity in the lights as well as the colors you can get a variety of effects without overtaxing Poser.  Key and fill are both infinate lights by the way.  Let me know if this works for you.  :)

Boni

Boni

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"Be Hero to Yourself" -- Peter Tork