Jimm 12/23/2012Several possibilities; coolant mixture is incorrect (not at recommended 50/50 water and anti-freeze), faulty thermostat, faulty / clogged radiator hoses, faulty water pump, clogged / restricted cooling passages in the engine or radiator, fan belt stretched or slipping.
gm3119 12/26/2012Had same problem with my 199 deville... It was the water pump. Get ready to lose money. :(
Jimm 12/26/2012Try the many on-line auto parts sources, such as; www.autopartswarehouse.com or www.rockauto.com for the replacement parts. For example, they (RockAuto) list the water pump for around $22.00 for your vehicle.