As the title suggests my PC tends to shut down from time to time all of a sudden, shuts down totally and when restart no error msgs,neither hard disk nor check of any kind. This problem mostly happens during playing games (after 5 mins or so after starting them,baring in mind that those same games used to work perfectly before), at first i used to un-plug the power (electricity), and then after a minute or so plug it on again,and then i could play whatever game that is for longer (30 minutes).. i formatted the pc,installed latest drivers,cleared CMOS and BIOS (Thought it might be the problem) nothing changed, and it's not graphics card issue because it even shut down couple of times when tried re-installing windows... i've read few other articles and the suggested problem was the heat, but i wonder can this be a total RAM problem, though it's a new 256 chip and it runs smoothly,just it seems when the PC needs more sources either RAM or CPU it turns off totally..
Any suggestions would be well appreciated, and no error logs related to the problem itself! i checked the eventviewer..

If the system is clear of dust etc and the fans are running OK, and this problem coincided with new RAM, remove it and go back to what you had and see if the problem persists. If it doesn't, the new RAM is possibly faulty, or it's not right for the motherboard.

Did you change any existing ram to a different slot when you added the 256 MB module?

The symptoms you describe can fit into a ram problem catagory.

As a double check, you can download MemTest86+, run the file to create a program floppy disk, and test the memory (if it's correct for the motherboard), preferably overnite. Any failures indicate faulty RAM.