Overheating radiator 92 SC300

[Sc300Rookie]

Hey guys I had had some questions about my overheating radiator in my 92 SC 300

First things first i just recently started working on my SC 300 this past year, it has 143,000 miles and the car was in poor condition with rusted brakes, rotors, and more. The car was running smooth for about a month after changing the fuel pump switch which kept causing the car to die on me every so often. After fixing all these problems and changing all the fluids I noticed one day that after driving for about 10 minutes I began to see smoke/steam coming out the side of my hood and my temperature gauge was extremely high, after I pulled over and opened the hood (with a glove) I noticed that my engine coolant cap burst off and the coolant seemed like a burnt dark brown color. My dad and I tried to put some water inside the radiator to replace the coolant. I test drove it for a bit and the same thing happened, I started seeing steam rise from the hood and it sounded like the water was boiling. I let the car rest for a week before my dad and I decided to flush the system completely with water and change the thermostat. So once I thoroughly flushed the system for a second time i took the car around the block and it was doing fine up until about 8 minutes in and I started seeing steam again. I have no idea what going on so any help would be appreciated.