Power cut in the system unit. What is the reason?

Question

Electronics

Bought components and assembled a small computer.
Mat. board: ASUS J1900I-C
Case + PSU: INWIN BP659
Hard drive: Seagate Barracuda 7200.14 ST1000DM003
RAM: 2x Kingston KVR13LS9S6/2
Initially installed Debian and set it up - everything worked flawlessly, even left it to download torrents overnight. After about 2 weeks, it began to spontaneously reboot - I thought it was nothing.
Found out why it reboots. As it turned out, the food is cut to the mat. fee, and since in the BIOS settings, the "Restart after power loss" flag is set, then it rebooted. Great, I thought, and decided that my hands were still crooked for using Linux - I rolled Windows 8. But the problem remained and everything got even worse.
Almost every time I tried to turn on the system unit, it worked for 2 seconds and the power was cut off. It turned on again, but it didn’t even reach POST (no signals), there was no image on the monitor. Once out of ten it started, worked for 10 minutes and turned off.
What did and did not help:
1. Reset the BIOS.
2. I checked the power supply at full power (I sinned on it in the first place)
3. I checked the RAM with MemTest.
4. I swapped RAM sticks, inserted one at a time and even took it from a laptop.
5. Turned off the hard drive.
The only thing that helped was replacing the power supply with another one (550W). But I doubt that such a system needs much power.
Help the fool, I've already tried everything, what could be the problem?

Answer 1

200W should be enough for your system. Send the PSU in for diagnostics.

Answer 2

You need to measure the voltage of the electrical network when reboots occur. Apparently your power supply is "afraid" of even minimal jumps.