suprima 100l?

i am trying to fix this boiler for a friend (his dads a plumber) i am a rgi the boiler runs up to temp (not over)and cuts out and doesnt restart it may need resetting a few times its under the stairs and doesnt have the required ventilation i so i opened the cupboard door the gas pressure is 14.8 mbar i have asked for these problems to be sorted before i can try to find the source of the problem the whole system had trv but i removed one on a large rad there is no boiler interlock and i think the boiler is over sized does anyone know of any simple common faults on the suprima 100l i know the system doesnt help but i dont think im the dads best friend asking for all these faults to be put right after all that i,d better be able to fix the boiler