Can MacBook Pro be used 18 hrs daily?
Sudden shut-downs I would always attribute to heat - dust/fluff collecting in the fans &amp; heat sink fins. It takes more than a quick run-round with a duster or a can of air to properly clean out the fins; it needs splitting open &amp; getting in properly - even so far as replacing the heat sink paste.
It's not a job for a beginner, especially on a laptop, but the guys at the Genius bar ought to be able to steer you in the right direction.