It might be that the computer is overheating (you haven't over-clocked it by any chance?).
Good products for keeping an eye on the temperature are the general-purpose SpeedFan and the GPU-oriented GPU-Z.
Something else to try :
Disconnect all cables and devices to the computer and power off. Disconnect the computer from the mains and leave it off for at least 30 minutes. Then turn it on and reconnect the minimum number of devices you need. If the power is through an over-crowded or old power strip (surge suppressor), you might want to try connecting it directly to the wall outlet, at least initially. Power it up with nothing connected, except for the power cord and monitor. Then, connect just the standard keyboard/mouse and use it that way for a while. If the problem does not recur, start adding your peripheral (USB/FireWire) devices; one by one if possible. If the problem recurs, note what device was added last.