Computer Instantaneously Shutting Down at Random Intervals

[BatteryPack]

My computer often shuts down instantaneously without warning. Sometimes it will stay on for hours, other times I cannot get it to boot. When it does shut down, when I attempt to power it back on, it will turn on for 1-10 seconds (sometimes it gets to starting up windows, other times it never even makes it to BIOS), beeps once (sometimes it doesn't beep at all), and shuts down. At first I thought it was a failed PSU so I switched my PSU from a 600w to a 550w that I had lying around. On the first start up, it beeped and instantly shut down (although now it is currently turned on using the new PSU - who knows how long it will last though). I have attempted replacing my RAM sticks one by one to see if it was RAM failure causing the ordeal - no dice. Is my motherboard failing? Is it my hard drive? I have absolutely no clue what to do or what to try.

 

[clutchc]

What processor and gfx card are you using? And what were the make.model of the PSUs you were/are using?