hello everyone

I am new member here, so please excuse me if some of my questions were
answered before or just stupid. 8) reading the archive on internet and
googling did not help much.

I have a combi bolier (modena 102) that keeps giving me grief. it is
getting worse - just a couple years ago it worked fine. the main problem
is that it keeps cycling, and it does not seem to do it right - water
temperature of the shower keeps fluctuating from really cold to very
hot. I tried to eliminate this problem by installing a new shower head
with better throughput, but it did not help. I don't think the flow is
to blame. the same is happening with hot taps everywhere in the house.

the service manual for the boiler states that it should modulate the
flame and I don't see that happening at all. I inspected the flow meter
visually and cleaned the inlet filter. the output HW temperature sensor
seems okay as well (~4KOhm). I may be wrong, but it seems like the HW
running is just too hot and that boiler's HW temperature regulator does
not work. perhaps the limit thermostat is cutting everything off to
protect the boiler.

is it possible that the gas pressure is to blame ? I can't check the
flow meter, since I have no oscilloscope.

any ideas that I can try before calling for professional to rip me off ?
I am pretty confident he'll say that I need a new pump. 8)

thanks very much in advance.

/max