Alpha cd24 Combi Boiler Problem water not staying constant heat

[sphinxy]

H i have a 4 yr old Alpha CD24 boiler and we are having a problem with it being that when we run the hot water it heats up but does not stay hot it runs for a while hot then lowly goes cold then heats up again if it does not heat up again i turn tap off for 30 seconds or so then back on and it does exactly the same thing hot to start then cold back to hot any ideas anybody

[The Builder]

Thermostats usually, sphinxy. Nothing you can do yourself...... time to send for ............ BoilerMan!