Peculiar Gray Screen Shut Down Situation

I've been perusing all across boards and sites for a situation that matches mine, so I apologize in advance if I completely missed the answer to my issue somewhere else.

I have a Macbook, 2007ish, and I recently installed Snow Leopard on it finally. It worked fine for about a month or so, but I began getting the restart gray screen on Shutdowns (not start ups, which seem to be the majority of gray screen situations). I hadn't ever seen this screen in the 3 years prior to installing SL, while on original Leopard.

I noticed it happened when I went to shut it down with applications running, though I had done it millions of time before on the original Leopard. I started making sure I had fully closed out of everything before shutting down and I didn't have any issues for a few days, but it happened again, with literally no programs running before shut down. I've looked at my system profile to check my RAM and they're both stated as "OK." I'm also up to date on all updates, to the best of my knowledge. Just ran the most recent ones today.

Basically I kind of have a mix of questions: is this signaling the beginning of the end for my trusty, never-before-had-a-problem macbook, a SL issue that I can remedy myself (hopefully without a re-install, since I lost my disk in my last move and would rather not buy another) OR should I just go back to Leopard to avoid any of these issues. (I still have those original shipping discs, and backing up and wiping isn't an issue for me, I just did it for the upgrade). On top of it all, SL seemed to be a little sluggish in general, not what I had expected.

Thanks for any help you guys can provide, and hopefully I'm not wasting anyone's time with obvious questions that have flown over my head.

Bump, just once. Could really use any kind of input. Thanks.

If you have your original install disks for the machine you can use them to run some tests. It sounds like your hard drive is experiencing problems. And since your machine is vintage 2007, a failing hard drive would not be unusual.

Boot the machine with your Leopard DVD (assuming the original is Leopard) and run a verify and repair of the hard drive from the boot DVD. See the following LINK to learn how to use Disk Utility.