CAUTION: This bug seems to be corrected from version 4.0.2 and above.
I noticed MUCH improvement after upgrading to newer version!!!
Seems there is a bug that will be fixed on upcoming builds of VBox 4 and it's the root cause of malfuncioning ICH9 on VBOX.
FROM vbox forum:
its a bug, you cannot reboot, you have to shut the vm down each time and start fresh
its fixed in latest code, but no build has been pushed as yet