Had a similar problem with a friends laptop earlier this year with the same symptoms. Upon checking the obvious power cords, power output from the power supply, my arm bumped the power cord going into the back of the laptop and the system shutdown. What this was was the power connector on the motherboard was loose/broken around the solder pads on the PCB. After replacing the power jack the system powered up and stayed powered with no problems. There appears to be quite a few laptops that experience this issue. John