Got same thing on mine. Ive flushed it a number of times, its got a bit better, but not a lot. Have a look at the insides of your coolant pipes - they'll be coated in orange muck, plus the inside of your rad will be the same. Try reverse flushing your radiator (take it out, turn it upside down, stick a garden hose in it and flush). you could also try a radiator flush, couple of quid from halfords etc...