Check the radiator if its clogged-up, water pump working properly if okay could mean a blown headgasket, cracked head and block or an air lock in the system.
Best way to find out more why its overheating is to get it block and pressure tested.
January, 18, 2009 AT 12:03 PM
DO A COOLING SYSTEM PRESSURE CHECK OVERNIGHT
REMOVE THE #6 SPARK PLUG AND USING A BORESCOPE SEE IF YOU HAVE COOLANT RUNNING DOWN THE SIDE OF THE BORE
MOST LIKELY ITS THE #6 CYLINDER WHERE THE HEADGASKET BLOWS
NO BIGGIE JUST MAKE SURE YOU HAVE YOUR BLOCK, AND HEAD CHECKED