The random shutdown issue is caused by the heatsink expanding and coming into contact with the logic board. This issue may take anywhere between a few weeks and a few months to become apparent as the logic board is insulated, but over time the insulation is melted away.
Once the problem exhibits itself, the Macbook must be sent back to Apple to have its heatsink replaced with a slightly different revision which does not touch the logic board when warm. In some instances, the logic board may need to be replaced also.
News source: In House
