thanks for your input guys but as I said mem test showed nothing wrong about my RAM memory. I got desktop my PC is Lenovo IdeaCentre H50-55 AMD Quad Core A10-7800, RAM 8GB DDR3, AMD Radeon R7 240 2GB, HDD 1TB 7200 , DVD, USB 3.0 windows 7 home premium. It is premade PC. And yes it restarts something like you just pressed restart button no bluescreen of death etc nothing.....as I said guy who tested my pc with every possible test (CPU, RAM, PSU, motherboard, GPU) he couldnt find anything wrong with HW.....we will see since he said to me he will try those tests for another 3 days and if it shows nothing again....he will try every part of my pc separately.