boiler works for a short period?

i have a worcester 28si mark 2 it fires up on hot water or central heating and works fine for a while then cuts out it tries to reignite but no joy then it will not reignite for about another half hour have taken the front panel off control panel area and have noticed there are 2 relays soldered onto control panel just under st1 connection when it ignites you get green flash from relays when the fault occurs the left relay seems to arc out inside, the other symptom seems to be looking through the viewing on combustion chamber have noticed that there is good flame but then the flame seems to die from the left of burner to the right