Virtual Box has a some known problems with CPU emulation, especially if Intel VT-x AMD-V is not available. If your physical CPU supports hardware virtualization, try to enable it on Machine-Settings-General-Advanced-Enable VT-x/AMD-V. This checkbox disabled when this extensions is not supported by your CPU. If this checkbox is enabled, check it. Without this Virtual Box cannot run any 64-bit guests. And without this even OpenBSD/i386 (and some other systems) runs with serious problems.
If this does not help try another emulator, like qemu or vmware. Virtual Box is still too buggy.
