ravenheat csi 85 problem?

hello,
my ravenheat csi 85 has just developed a little issue .
What happens is when the ch is off i turn on a hot water tap and the burner kicks in and heats the water ok but as soon as i turn on the cold water tap the burner turns off. my cold water supply is producing 20 liters a minute, hot water on its own is pumping out 9 liters a minute and when the cold water is turned on the hot water drops to 7 liters a minute. Any one have any ideas, is it liable to be a blocked diverter or secondary heat exchanger?thx in advance