You experience the following symptoms on a computer that is running Windows Vista:

• The Allow hybrid sleep option is missing from the Advanced settings tab of the Power Options dialog box in Windows Vista.
• When you run the powercfg.exe -a command to list the power states that are supported on the computer, the output that appears includes the following lines:
  

  The system firmware does not support this standby state.
  Hybrid Sleep

These symptoms occur intermittently. When you restart the computer, the Allow hybrid sleep option reappears. Additionally, when you run the powercfg.exe -a command again, the output that appears includes a line that resembles the following:

The following sleep states are available on this system: Hybrid Sleep