If the PC had worked before as in it would correctly wake-up, then investigate any recent changes. Was there a s/w update or addition. Any h/w changes or additions to include drivers or related s/w. Also, you may want to disconnect any USB devices to help test here. If the problem goes away after such action, then an arrogant USB device or some corrupted s/w is at work here. Many times some USB device can cause a problem but is taken for granted it isn't so don't dismiss it too readily. Be sure no backgrd. running task is very active when put to sleep, it may not want that, thus verify what setting may better allow it to work when the PC decides to hibernate, etc., because it may need a fine tuning or visit again as an update reset some setting. You suggest you put the PC to sleep on demand??? Why not just allow it to go on its own and check results.

Also check you activity/event logs to see what is the last action or process that may have caused the issue to arise, review it.

http://windows.microsoft.com/en-US/windows-vista/Open-Event-Viewer

tada -----Willy Happy