1996 Deville northstar engine

Hello purchased car for verry small amout have all of the servicerecords. Previous owner had rad. Thermostat all hoses replaced plus numerous other repairs done at the same time at the local dealership at a cost over 4800 dollars when the car was returned to the owner it started overheating. Owner was informed tnat a new engine was required this is how I came about buying the car so cheap heres the situation, temp will spike and then cool down until you start pulling a hill it doesn, t matter how steep, engine goes to 265 degrees, coolant level light comes on and you are informed to shut the engine off. Car cools off rapidly .I perfomed a rad leak test and a combustion leak test passed both test. Fans run 5 to 10 s econds at a time, engine runs great , no internal noises help lrac55