OPTIPLEX 7010 random shutdown

I've been using Windows 10 since last year without any issue

The OPTIPLEX 7010 is using GTX750 Ti and Samsung EVO 500G SSD for months.

Strangely since a few days ago there's a sudden shut down problem. 

1- When the PC is in sleep mode, sometime it didn't got back from sleep, instead its a new startup.
2- When in Windows 10, there's random shut down (the screen display "shutting down" like it is a software process). I already disabled "power down" function upon pressing the button.
3- When in Windows 10, sometimes it simply shut down suddenly (like a power cut)
4- When starting PC, pressing the Power button on the case, sometimes the light fade in for like 2 sec then fade out (amber -> white -> off ) and powered down immediately. Keep trying and it would eventually power up (SSD takes 5 sec to load windows 10).

For last week the problem is minimal, I can start up the PC and it can run for hours peacefully, most of the time. Today the problem got more severe, I got at least 6 or 7 shut down, some happens within 10 mins of running Win 10.

The diagnostics at BIOS does not find any issue, fans are normal.

I can't figure out why this is happening, seems like a hardware issue (power supply failing?) but why sometimes it is Windows 10 shutting down itself (soft shut down). Any answer will be greatly appreciated

Thanks