Unstabel/non-booting system (MSI 890FXA-GD70)

Hi! My system sometimes simply shuts off while running or during boot. Please help me narrow down what's causing this before I start replacing everything! I'll give you the symptoms and the specs:

Specs:
MSI 890FXA-GDA70
AMD Phenom II X6 1090
Noctua NH-U12P SE2
2x 4GB G.Skill DDR3-1600 Ripjaws
Enermax MODU 87+ 700W
MSI N460GTX Hawk

Symptoms:
The system is custom, but not newly built. It has been running smoothly for a year.
Now it sometimes shuts down without any warning signs. When I (re-)boot it sometimes shuts off directly, within 1 second.
I think the last LED DEBUG code it gets too is FF.
During start all fans and all LEDs start up normally. When the system shuts down the hdd LED (both on the mainboard & on the case) keeps blinking regularly, 2 DDR phase LEDs remain blue and the fan of the PSU keeps running. Everything else is dark/shuts off.
This doesn't happen all the time, sometimes the sytem works fine for hours or days. That makes it difficult to troubleshoot since I can't be sure of any effect right after I changed something.
So far I've only performed a CMOS reset and checked all cables. I guess I'll have to start to change components, but I'm not sure which. Mainboard, CPU, PSU, RAM?

I urgently need my system for my job. Any help would be appreciated!

Chances are that it's either your power supply or ram. More likely the power supply.

Unfortunately, swapping out equipment is the only way I know of to check on the power supply. You can run memtest86+ to check on the ram though.

After running continuosly for 32 hrs with the new PSU I'm beginning to believe that the problem has been solved. Thanks again for the suggestion, abekl.
It seems I wrongly suspected the mainboard, so apologies to the forum (and MSI) for placing the thread here! Aaaand I just noticed that I titled it unstabel instead of unstable. Oops