ok, most probably ur lid sleep sensor is not working well.
you can try to disable lid wake.

go to terminal and type:
sudo pmset lidwake 0

to enable it again, change zero in the end to digit one.

I found this tricks extra useful. I wont have problem as my powerbook accidentally wake inside my backpack because I didnt close the lid nicely. I can also open it up for cleaning without waking the machine.