Ariston boiler continually locking out.

jandap

Hi,
Will try to keep this as brief as possible. Ariston boiler Type C G.C.N 41-116-06 or 41-116-07. Boiler has been installed for 6 years, as winter is coming decided to have it serviced. The engineer (Corgi registered and reputable) came, dismantled part of the boiler cleaned it, re-assembled and tested. After 1/2 hr boiled locked out, pressed reset button, started, after 15mins. locked out. Locks out intermittently. Since then two PCB's have been replaced, still locks out, all three electrodes have been replaced, still locks out.
The plumber and myself are at our wits end, tomorrow he is going to try the techie people at Ariston. If anybody has any ideas they will be most gratefully received. Perhaps I should have abided by the old adage, 'If it 'aint broke, don't fix it!
In hope.
David