1998 Cadillac Deville what else is there?

Engine Cooling problem
1998 Cadillac Deville Front Wheel Drive Automatic

We have a 1998 Cadillac DeVille with 98k+ miles. We keep having problems with overheating. We have replaced the thermostat, a cracked radiator, the top radiator hose and the radiator cap. Checked the water pump, it's pushing great. There is no water in the oil and no oil in the water. We also used a product to seal a blown head gasket. We are at a loss. What else can there be? We bought this care from our landlord who also has a buy-here, pay-here lot and we have put more money into the car ht we have paid on it. We can' keep pouring $ into it forever and not be able to drive it more than a few miles before it overheats.