bit more technical detail wouldnt of went a miss but my guess would be scaled/sludged up exchanger giving crap transfer of heat to passing water (your hot) but staying very hot in the boiler circuit so it cuts in and out.
iain
May 2008
I can answer this question...
hot water not hot just get warm then cold then warm?
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.