Macbook shutting down suddenly

Hi everyone, having a problem with my Macbook (10.6.8). Last night while watching you tube, it shut down completely. When I went to reboot, it shut down again in the middle of doing so. Finally came up and was working fine. Just shut down again a few minutes, again while watching a video. Could this be a Flash problem? Someone mentioned it could be an overheating issue, but I would assume I would hear a loud fan noise it it was, which I am not. It doesn't seem to happen at any other time except watching videos, so I have started to download anything I want to watch.

Thank you for your help.

I just got done repairing a 2009 Macbook that had the same issues. It would just shut down - no warning. It would do it just sitting idle or running a program. There was no consistent connection that indicated a software issue. I would have to push the power button and wait for it to go through a cold boot.

So I finally broke down and opened it up removed the heat sink to replace the thermal grease. I also cleaned the fan out. That seems to have fixed it. I had it capturing a live performance tonight for about an hour and it went fine. I also left it on all night only letting it shut off the screen and that went well. So hopefully it's fixed. The grease was dried out so I am sure it needed replacing.

If you need instructions on how to go to ifixit.com and look for your model.

Lisa