Water runs hot and cold boiler makes a noise as if there is air in it?

I have a turbomax boiler, the heating runs fine and isn't a problem, when i turn the hot water on it will run hot for about 30 seconds and then run cold, it will do this for about five mins before settling down to cold water only. In addition the boiler makes noises as if there was air in it, i have drained the boiler several times and refilled and bled but this noise persists now and then. Also when the hot water is turned on the boiler sounds like a kettle boiling. Could this be a diverter valve ?