Recently I've had a problem where if I try to open World of Warcraft it would look like my computer went into sleep mode(happens maybe 50%-75% of the time). The monitor loses connection and flashes an amber light while the computer still runs. The only way to fix this problem is to restart the computer. Eventually the monitor still would not gain connection even after a restart and the only way I found to get it back would be to unplug the cable to the power supply in the back and reattach it. After doing this for maybe a week, one day I turned on the computer, but it looked like the motherboard wasn't getting any power, only the harddrive light would light up on the case and you could see the cpu fan wasn't even running. So after unplugging and reattaching the power supply cable it worked. Now I'm left in a situation where any of those things could happen at any point.
I can't figure out if it's the power supply or the motherboard or something else causing this problem, and I'd really like some opinions before I go and buy a power supply and still have the problem.
I'd appreciate any help or insight on the subject.