Prophet1000

Hi, My PC has started shutting down by itself, is it just bored or is something about to blow? To restart I have to switch off at the mains first. Sometimes it switches off within a minute of switch on, sometimes it will last a couple of hours, and sometimes it waits for me to switch it off when I've had enough. I put in a 1GB RAM stick a few weeks ago with no problems until now, is it connected to the problem? I checked compatability before purchasing. Any help would be gratfully received. Martin