I upgraded to Kubuntu 15.10 couple of days ago and experiencing weird freezes. When the freeze occurs, the screen is unresponsive but I can move the mouse. It occurs pretty randomly - when using the terminal, surfing etc. I went through the logs and haven't noticed anything. I'm kinda suspecting problems with the compositor/cards but I'm not sure how to proceed. Using a Dell inspiron with Kubuntu 15.10, Hybrid Nvidia-intel card (using nvidia 352 through driver manager)
Had similar problem with 16.04 and 16.10 - the box stopped responding to keyboard and mouse. Switching to Compositor settings to XRender seems to resolve it. System Settings -> Display and Monitor -> Compositor -> change Rendering backend to XRender
I've got the same problem, but without a Nvidia card. It's the Intel integrated GPU. Killing Xorg solves the problem, but it's obviously not a solution.