how can i fix my baxi 105HE?

my boiler water pressure keeps on goin up to nearly 3bar and expelling alot of water outside.CH works when i drop the pressure down to 1 using the pressure relief valve which leaks only while i drop the pressure.( joint leak) there is only about 10 seconds of hot water after this but the heating is fine till the pressure gets too high. i am regulating the pressure daily.there are no flashing lights on the panel. i have noticed that when the CH is on, the neon is lit normally but once i turn a tap on it goes out and the DHW light comes on instead (not both together) but no hot water. diverter problem perhaps?