VirtualBox suspends execution of Windows virtual machine when close to idle

Question

I have VirtualBox under Linux and I run Windows virtualized in headless mode.

When the virtual machine is close to idle, it seems the VirtualBox suspends its execution, so my scheduled tasks take some unusual 30 minutes delay to start. The clock sync is done through VirtualBox guest additions. What can be wrong?

Windows Guest: Windows XP SP3, Energy options: always on, no hibernate, no suspend, no screen saver;