Usebackground Material comes out grainy in render
 

Hello All
Please help
I have a scene that I used a "UseBackground" Material on a nurbs plane. but when I render them in mental ray or in software renderer that plane show up in render grainy... the backgroung image is showing trough it but alot of gray spots are on the plane...
would someone please help me in this case?

could you post up a screenshot or render soo we have a clear idea of what it looks like :)