[Q] Phone Shuts down automatically

Got my Zu a few weeks back. When in stock, this problem occurred randomly and extremely infrequently. Now, the phone is unlocked and rooted and currently in 757 (4.4.2). Now the frequency of the problem has increased many fold. Whats the problem? ... The device just switches off without any notification or alert!! Even when fully charged. I need to switch it back on (with the power button) when I realize it has switched off.

This switch off randomly happens at-least 3-4 times a day. I used the "catlog" app to see if any errors or failures are logged (which may cause the shutdown), but the log entries are so many and so many are generated that they just fly past for me to figure out anything. Is there any specific error that I may have to search for? Any ideas on how to approach the problem and maybe solve it? If it turns out to be a h/w issue, I can send it for service. But I need to ensure that it’s a h/w issue Any help will be greatly appreciated. Thanks!