glow worm swift 80 - not keeping a constant temperature?

Hi

Whenever we have a shower - it does not maintain a constant temperature. We can get boiling hot water - but on adjusting the temperature the boiler seems to cut off and the water goes freezing cold. Also, on the hot water tap in the kitchen I've noticed it comes on fine to start with but then the flow rate decreases to almost nothing! Any ideas on what the problem could be? Thanks in advance.