MacBook restarting on Standby mode

Here's the backstory on this laptop first:

Purchased November 2006. Never had a problem until the end of last month (Sept. '09) when the logic board crashed.

Apple Care replaced the Logic Board and the optic drive (unrelated problem).

Since the replacement of the logic board, the computer has developed a bad habit of rebooting whenever I close the lid to put it into standby mode.

Has anyone else encountered this issue, and if so, what was the fix? I'm getting really irritated with losing work when the computer goes into standby.

Thanks in advance for any help.