I would highly suggest you replace the radiator, if it is rusty and leaking there it could be untelling what kind of shape the inside of it is in, no matter how many times you clean it. I had a similar problem such as yours on a 93 coupe I had one time, a new radiator solved my problem. A new stock replacement is not that bad expensive and is a easy swap, the radiator, water pump, clutch fan and the fan are the heart and soul of the cooling system. If you are confident everything other than the radiator are in correct working order then replace the radiator.