Even if your power supply passes the paper-clip test that Panicmaster links to, I would still suspect it. I had very similar symptoms a while back and the cause was my graphics card not getting the required power from the power supply. I replaced my 450 watt Cooler Master power supply with a 500 watt Antec and my problems disappeared. However, I would probably invest in a power supply tester
before scrapping your current PSU.