Unstable - computer turns off and on again in a second

[master9tom]

I have actually sort of, but not really, fixed this problem. I ran my computer brilliantly for a few months, on a 750 ti, and about 6 weeks ago I upgraded to a GTX 960. It ran perfectly, until yesterday when it couldn't find my boot medium (I run my OS of the SSD), but it could find my hard drive. I opened up the case, and moved the SATA cable into another slot. I turned it on again, but the fans span for a split second and the lights went on, and then it went straight off again. It did this again and again, and I had to turn this off with the PSU switch on the back of it. I looked through the forums, and although I found a lot of people with same problem, none of the solutions worked for me. Then I read the guide before posting, and I moved the RAM to the slot furthest away from the CPU, (at this point I had taken away the GPU and Storage, and taken it out of the case to remove any chance of faults), and at first it did the same thing again but then It started up normally. I turned it off again, and then plugged a video cable into the Motherboard. It went to BIOS. I turned it off and tried it again. It did exactly the same thing. I tried a few more times, and again it went to BIOS. I've checked multiple times, and all my connections and RAM should be in correctly. What should I do, as the system is currently so unstable?

Specs:
CPU: Pentium G3258
GPU GTX 960
PSU: Corsair CX500
RAM: Kingston Hyper X 8gb
Motherboard: MSI H81M-P33
SSD: SSD now v3 120gb
HD: Western digital blue 320gb
Case: NZXT S340

 

[Dark Lord of Tech]

Could be a power issue.