Riello rdb 3 burner cuts out?

About a week ago woke up to find the rcb switch in house had tripped. Narrowed it down to boiler which is outside. strange thing is when reset i started the burner again and ran fine for ten minutes then tripped the rcb again. Reset rcb and started burner and this time it only ran for 30 seconds and rcb tripped. have had electrician out and he isolated the power from timer to burner (took power from timer through extension cable to burner and ran fine! So I presume its the cable going from timer, underground and running outside to burner? the burner was re connected to its proper cable and then wouldnt fire at all! Eventually(after the electrician hit reset button a few times) it fired but this morning nothing again. iwas told to keep trying it as this can possibly rectify if the cable has only very small break/short? something about the arc from possible short building up a mini weld and repairing itself? Doesnt sound too viable to me but hey, me simple minded artist. any ideas?