fireing up and then shuting down it gets up to heat ?

hi
it keeps firing up shut down in seconds
ho yes i have also put in line a expansion chamber
its been doing this for weeks it all started when i was not getting h/w so i cleaned the h/w heat exchanger now h/w is good
and the rad's get hot very quick it all pumps round that's all good but when the are up to heat the boiler still keeps a high flam then shuts off in seconds it's back up high flam i am thinking the pcb board / help

DAVID MCCALL
March 2011

how long has it been like this and how often does it go on/off

brian
March 2011