Then you are most surely having the problems outlined in the discussion that I linked to.
I had similar issues. I had AppleCare. The logic board was replaced gratis.
So far - after nearly 3 months, no more problems. But some have had their logic baord replaced 3-4 times. The only real 'fix' seems to be to have a new GPU expertly replaced. There's a California company that has a years guarantee on their work.
If I were you, I would take the machine into your local Apple Store or AASP and have them run diagnostics. Your board may pass all tests (mine did) yet the problems persisted. So the logic board was replaced. If you have warranty left, your logic board will be replaced free of charge as well. If you're out of warranty, I would advise getting the GPU replaced rather than paying for a new logic board.
Good luck,
Clinton
EDIT: It's NOT a sreen problem, it's a GPU problem.
Message was edited by: clintonfrombirmingham