the flame on my modina 102 boiler cycles on and off?

the flame on my boiler keeps going on and off every couple of minutes and also the water in my flat goes hot and cold every minute.

I had a gas man out and he checked everything, he cleaned the fan, checked the sensors, checked the heat exchanger and serviced the boiler. He said that he though there was a problem with either the flu being too long (it's about 12ft or that it might be broken somewhere because not enough oxygen was getting into the boiler from outside. He proved this to me by showing me what happened when the front cover of the boiler was on, the flame kept going on and off and the water temperature kept changing. When the cover was off and oxygen was getting in from the room the boiler worked perfectly, with constant running hot water.

I'm still not sure though that the problem is due to the flue, could it be due to anything else because I don't want to have to pay a lot of money to have the flue dismantled if that is not the problem.

Thanks in advance for any advice anyone can provide.


Denise