iMac Restarts when set to Sleep

I upgraded to Leopard last Wednesday. All appears to be working well thanks to the iMac software update Apple released late last week. One annoying problem has introduced itself since the upgrade though. When I set the machine to sleep it restarts the computer. Sometimes it'll take several hours but other times it'll be sleeping for a little as 1 hour and then restart.

I've read around that it could be the RAM I have but the problem only started after the upgrade to OS 10.5 and I installed the Gig of RAM in early September...

Has anyone come across this before?

I've woken in the middle of the night three times this week to find my Mac awake. This is apparently a bug, with lots of posts on the Apple Discussion forums. Best thing to do is report it to Apple via the following link:

http://www.apple.com/feedback/macosx.html/

Thanks for the reply but I believe I found the solution. It appears my RAM was not pushed all the way back into its socket. I pulled both 1 Gig sticks out and pushed them back in ensuring that they were fully back. This appears to have corrected the issue.