Power off randomly after screen lock... I have a very interesting problem. This is a chinese based phone Alps i4000 (fhd, inew i4000) with Jelly bean 4.2.1 os. 

The problem: 
 - After i click the power button to screen lock, the phone randomly turn off (random value from 1 sec to 15 min)
 - After i let to go to automatic screen lock, also turn off randomly the phone. 

But if i use / let running any software, its working well. No battery problem, no temperature problem.

the phone stock, no rooted, no custom rom, no custom app (apps just from play store). 
Bitdefender and avira antivirus say everything okay. 

Idea?
Sounds like a hardware problem to me, like the internal power supply. Fixing would probably require a change of the logic board.

If the device is still under warranty, I would just return it as defective.