After several days of this problem not returning, I booted yesterday to my Xubuntu 64 bit, fully updated with a new kernel 3.13.0-19, but as a virtual machine in VirtualBox PUEL version 4.3.8, with Guest additions installed and otherwise running beautifully, and there it was again.
I copied the three logs you requested and I attach them here.
After several days of this problem not returning, I booted yesterday to my Xubuntu 64 bit, fully updated with a new kernel 3.13.0-19, but as a virtual machine in VirtualBox PUEL version 4.3.8, with Guest additions installed and otherwise running beautifully, and there it was again.
I copied the three logs you requested and I attach them here.