There is an issue that many mobile and tablet users have encountered with Windows Vista, myself included. Warner Crocker also has encountered this issue and describes it thus:
I’m still frustrated with Vista on one front here and that is putting the Tablet PC to sleep. 3 times out of 4 everything will work as advertised. Close the lid, unit goes to sleep, open the lid, unit returns. But occasionally the OS just goes off into its own world without sleeping, or after a successful sleep, not returning. Occasionally it will return from sleep but the screen will stay dark.
I personally believe that this issue is not a Vista issue per se, but rather a driver issue. I think that some drivers are poorly written and interfere with the sleep and resume process. I also think that this issue is much more serious than it initially appears. I have come to refer to it as the sleep of death. But that I'll leave for another post.
For Warner it seems his issues are related to when he is using Wi-fi. For me it was BlueTooth. Either way it points at drivers.
I’m beginning to think (this seems to be somewhat reproducible here) that these issues have something to do with being connected or disconnected to our WiFi network. Here’s what I’m experiencing and maybe someone smarter than me can help out here with some thoughts.
What worked for me was to disable the Power Management features for the affected driver. For a wireless driver you would do this by going into Device Manager, right clicking on the relevant device and selecting Properties.
You may then see a Power Management tab. Not all device or drivers implement this interface, so if yours does not you will not see the tab.
If there is one select that tab and then clear both of the checkboxes shown in the screenshot below.
I have done this for my LS800 and it has all but eliminated the dreaded sleep of death issue. I use to encounter this several times a week and now I get it less than once a month.