I have an Alpha 900 combi boiler (pressurised system, no header tank) which has been working fine for about 5 years. Recently the hot water it has delivered has been cooler than in the past and the temperature varies from warm to cool while in use. This is especially noticable when showering.
If I sit by the boiler when hot water is being drawn I can see the burner flames varying in strength over time in synch with the temperature changes.
Other symptoms:
The boiler now makes a faint moaning sound when in use.
The Radiators get hot (much more noticably so) when water is being drawn.