I tried disabling hardware acceleration in Google Chrome but that didn't help. And the system was freezing every few minutes/hours. So I went back to using Nvidia proprietary driver ( nvidia-364..which is the latest as of today). The computer no longer freezes. So I'm guessing either the Intel HD Graphics hardware is bad on my motherboard or there is a bug in Ubuntu support for Intel HD graphics.
These are the reasons I tried to move to Intel HD Graphics:
My second UHD monitor was not being detected after login. In nvidia settings it showed up as disabled. Enabling the monitor in nvidia-settings-manager would not persist after a reboot. I deleted my
~/.config/monitors.xml file and also used the Ubuntu Displays utility to switch the monitor on. The displays utility asked me if I wanted to keep this configuration and I said Yes. This seems to have fixed the problem.
The other problem was that monitors were not waking up from sleep. I had to reboot the system everytime monitors went to sleep or remember to manually switch off the monitors everytime I stepped away. This is not a problem with the Intel HD Graphics so I'm guessing there is a bug in the Nvidia Proprietary drivers. For now I have disabled sleep on the monitors and hopefully will remember to switch them off manually whenever I need to.
Hope all this helps someone.