Cadillac Deville overheating

I just bought a 1992 cadillac deville and it had an overheating problem. The radiator was cracked so I replaced it and then replaced the thermostat, there is circulation, the water pump is working and the thermostat is working, did a test to see if I had a head gasket problem and nothing. The previous owner I had noticed had put alot of stop leak stuff in the cooling system to stop the radiator leak and im out of options except I am going to try to chemically clean and flush the system but after that I dont know what else to do. I have a diagnostic tester hooked up at all times so I can monitor the coolant temp and while driving its been getting up in the 240 range. There are 143,000 miles on the car and its a 4.9l v8 front wheel drive. Any suggestions? Thanks!