Does it freeze with Compositing disabled ?
Your problem *could* be related to the broken "UseEvents" feature in Nvidia Driver 190.xx series. Open a System Monitor and see if the cpu get pushed to 100% during the freeze. You can edit xorg.conf and add:
Option "UseEvents" "false"
(or "true") to the Device Section, try what works for you. Read the Nvidia Driver Readme file for further Information. There are several threads on Nvidia Linux forums about this freeze problem in KDE. If you have the freeze with the old 185' Drivers its something different in your System/Config ><