Because you are using the Hot water faster than the boiler can produce it. Turn the tap/faucet off a bit. Get the proper sized boiler
Usually on the top, on the incoming cold pipe.
The flow of flue gases in chimney due to difference in pressure i.e cold atmospheric air outside and hot gases in chmney is called as boiler draught.
If the boiler is up to temperature, then the problem is with the pump. If the boiler is cold you should call a pro, as there is no way to diagnose it from here with out more info.
No. Hot water directly from boiler is not potable.
hot and cold combined.
new boiler and hot water heater
it should start at cold
combi
This depends on whether the hot water comes from a combination boiler or from a hot water tank with a header tank feeding it. For a combination boiler the fault would probably be at the boiler, possibly from scale in the heat exchanger. For the system that uses the header tank and a hot water cylinder it could be that the float valve in the header tank is stuck an there for it is not refilling the tank or debris has entered the tank and is blocking the outlet.
The fuel does not matter, it would be more relevant to post if you are hot water or steam, that is where your problem lies. If steam try raising the boiler cutout pressure a little bit and if hot water look at the boiler pressure, does the pump work, is the coupling broken and do you have air in the system that needs to be bled?
If you are using water and not steam there should be a temperature controller mounted in the side of the boiler, that is where the start and stop point of the boiler is controlled.