Is the radiator plugged up? Is the thermostat working properly? Rust in the system is not good and can plug up a radiator and coolant passage ways in the engine which would cause your problems. I'd try reverse flushing the system after adding a cooling system cleaner [following the directions]. If that doesn't work consider replacing the radiator or having it tanked by a pro and cleaned up. You might find replacing it, if you can DIY more economical. If you replace the radiator replace the thermostat, and chemically flush the system before you put the new radiator and thermostat in. Disconnecting the heater hoses and flushing the heater core would be a good idea too.