it is an overheating prob or power supply. take the case side off and look to see if the fan on the cpu is running continuously. If the fan is running but with the side off the problem disappears then you have a heat build up prob poss solved by repositioning the pc or by fitting an additional case fan. If the side off does not cure it then it may be the power supply starting to fail. This would not generate any messages before it restarted but you would get the recovered from serious error one afterwards.