Heating & DHW come on intermittently ?

I have a Procombi boiler (installed 3 years ago), It has developed a fault on it. When switched on from cold (selector on C.H & DHW) it runs for afew seconds then turns off,afew seconds later restarts,runs for few seconds and turns off again,this continues for 20 to 25 minuits, then it runs for longer i.e 30 to 40 minuits without any problem. we were told the D.H.W heat exchanger was faulty.the engineer cleaned the D.H.W exchanger with some chemicals, it ran ok for 2 weeks and the problem has returned. I am not convinced it is the Heat Exchanger because when it runs after shakey start there seems to be no problem,so a blocked Heat Exchanger should not clear up and then get blocked again overnight. Any suggestions any one?