The boiler overheats and then the high temp safety device cuts in
Take it it is doing that during hot water operation.
Put the preheat off and see if boiler still doing that.
Does it do that all the time or randomly.
With hot tap running, what temperature is the temperature gauge showing
Reduce the thermostat setting and see if that makes a difference
Look at the flame when hot tap is running, ( get some one to gradually reduce the flow rate). Flame should reduce in height. As the hot water flow rate is increased, burner will go to full rate.
The temperature gauge, with hot tap at max flow, should indicate around 70 degrees. The O/H stat chops the drive at 95 degree or so
If the flame size does not adjust to load ( heating of cold water), then of course O/H stat will kick in.
Checking temperature of primary water in and out of the main heat exchanger would assist in fault location. Data logger could also help narrow the issue.
Final drive for burner modulation unless I am mistaken is 2N3055 power transistor.
You need someone who has worked on this boiler and understands why makes it tick.