pc turns off on its own, turns ON on its own

[chesss]

The problems I am having are:

1. PC would turn off randomly instantly (no overheating , I checked)
2. Sometimes It would partially start on its own, when the UPS is turned on. By partially I mean the cpu/chasis fans spin, but the led power indicator light doesn't turn on .
3. When attempting to start the PC by pressing the switch, it would seem to start normally (fans spinning, indicator light on), go the os selection menu, then whenever the OS was selected (windows/ ubuntu) it would restart. Other times it would not reach the OS selection menu and instead be stuck in a loop.

I am at a loss here. Can't figure this one out. Help!

Config (6 years old)
AMD X2 4000+
ASUS m2a-se2+ mobo
1gb corsair , 1 gb strontium
amd 4670 1gb
seagate 500GB HDD
local PSU/cabinet

Thanks

 

[denis280]

first i would start with a clear CMOS. and did you make any change lately.

 

[chesss]

The current status is : I turn on the ups and the fans start spinning, no led light or anything on the screen.

Tried clearing CMOS - same effect.

 

[Ketchup]

Remove the UPS from the equation. How does the PC start plugged directly into the wall?