So this is exactly what I wrote. Max consumption per GPU alone is ~250W in their test it is 243W.

Sorry for telling you, but this is crap PSU.(http://www.e-shop.gr/heroichi-hc550te-550w-green-80plus-retail-p-PER.700118) Only 28A on 12V, so it can only give out 336W so no wonder that under full load (GPU 250W+ CPU and other devices 100+W, together 350+W, but this PSU can give only 336W) it will restart your PC.
Try buying for example XFX 450W, that has 34A on 12V, so much better than this crappy never heard of Heroichi.