virtualbox.org

I have Windows 8.1 64bit guest running on OS X Mavericks.

I have a problem whereby I often have to stop my VM because (usually after the host wakes from sleep) the screen starts to flicker and then eventually go black. The VM is still running, and if there are no programs stopping it, I can gracefully shut it down via ACPI shutdown.

Sometimes, when it flickers, the Windows 8.1 machine goes back to the login screen and seemingly forgets about my login session and all the programs I was running. It's as if the machine was rebooted. But usually after I log in the screen starts flickering again and eventually goes black.

It doesn't seem to matter if I am running in a window or full screen.

I have turned off all acceleration and that seems to have no effect.

If I pause my VM before sleeping the host, it seems to help.

This problem usually only happens after waking from sleep, or after a couple of hours of constant use.

Once, when the screen went black, I could see the screen again while resizing the window. I could also see the screen (grayed out) when I paused the VM. But that only happened once.

This has been happening ever since I installed Windows 8.1 64bit.

First, it's not a good idea to let your Mac go to sleep with a guest running as you found out.
Second, OSX 10.9 has two features, App Nap and Power Nap. Power Nap should be disabled when running guests. App Nap is a feature to put apps to sleep when they aren't being used but are still open. To disable App Nap for VirtualBox, right click on the VirtualBox app, choose "Get Info", then check "Prevent App Nap" in the General section.