ADVENT LAPTOP SHUTTING OFF !?

DOES ANYONE HAVE AN ADVENT LAPTOP THAT SHUTS OFF ? IF YOU DO PLEASE TELL ME .. OR TELL ME IF I COULD STOP THIS PROBLEM ? AND MY LAPTOP IS NO LONGER UNDER GUARENTEE. I HAVE HAD THIS PROBLEM BEFORE I SENT IT AWAY AND I GOT IT BACK , WITHIN A FEW MONTHS IT WAS BACK TO ITS OLD SELF ! SHUTTING OFF AGAING ... HEELP !