Flickering black screen when opening a window/program

[Sad Nation]

Hi all

I have been trying to speed up my Ubuntu 15.10 virtual machine by turning on the 3D acceleration in Virtualbox settings. When I do, Ubuntu loads normally but I get a black screen flickering 2 or 3 times before a window or program opens. It looks like another window is still open underneath but I can't tell what it is. Once it has opened everything looks normal. If I turn off 3D then it all works as it should, just very slowly.

Guest additions installed.
Virtualbox 5.0.16 r105871
Ubuntu 15.10 Guest
Windows 10 Host
2 Gig memory on Guest
2 Virtual Processors
128MB Video Memory

Thanks in advance for any light you can shed on the issue!

Sad Nation