For me the problem disappeared since I upgraded to the mainline kernel 4.8.9-040809-generic (my random pick) from here
http://kernel.ubuntu.com/~kernel-ppa/mainline/?C=N;O=D
as suggested by PeterPall above. Ubuntu runs like a charm, I only had to update my VirtulBox to the 5.1 series.
For me the problem disappeared since I upgraded to the mainline kernel 4.8.9-040809- generic (my random pick) from here
http:// kernel. ubuntu. com/~kernel- ppa/mainline/ ?C=N;O= D
as suggested by PeterPall above. Ubuntu runs like a charm, I only had to update my VirtulBox to the 5.1 series.