baxi 80/105 he when i turn on more then one hot water tap the others go off?

i have a baxi combi boiler 80/105 HE RANGEthe problem is when i turn on hot water tap ,and another hot tap one will go off .it seem to be no presure for the hot water taps. ihave no neons faults flashing.i have c/h is ok/the pre heat works ok ,bar is 1.5/2.00 .the 3 way asemberly is making a funny noses .i down load baxi manul no help .it has only just gone like it .it seems to only on the hot water taps, i have good mains presure .any idears