KVM / Gnome-Boxes / SPICE : Losing GUI connection

I have some vm’s running on a R8 workstation. Graphical access is done with gnome-boxes (spice protocol). I am running continuously into the situation that the graphical connection gets lost (mainly with vm’s based on R9). For example; boot vm, gdm shows up, when I wait to long (~5-10 minutes) the gdm interface disappears. Albeit the vm is still running and ssh into it is possible …
Does anyone have the same behaviour?