I'm running Vista 64, on a Gigabyte GA-MA78GM-S2H, and when I try to go into sleep or hibernate mode it reboots instead.

It isn't a standard reboot however, since at boot I am prompted with the screen telling me that Windows was not properly shut down. The problem also seems random, given that there are times where it goes into S3 sleep just fine.

Could this be a power/power supply issue? I'm running a 430 Watt PS, with a 4670 card, tuner card, 2 hard drives, 2 dvd drives, and 4 sticks of ram.