Check your overflow bottle diaphragm. If it has perished, you'll slowly lose coolant and then you'll have a overheat issue from lack of coolant return. Next is a chemical flush. I would remove the radiator and fill it with a batch of citric acid powder dissolved in hot water (only have to leave the cap on and have it lying flat with the inlet/outlet facing upward) You will probably need to drain, flush and repeat the process a few times (if you have pressure cleaner, this will help will freeing scale and rust in the cores and bottom tank). Give the radiator a decent coat of black epoxy paint too. It does help with heat dissipation.