Ok back to basics, Does it have Water AND Anti-freeze? You need antifreeze especially in the winter but many people dont realised it contains anit-rust agents AND doesnt boil at 100degrees, When water boils it has airbubbles in it which do not help the cooling and less water it physically in there.
Remove thermostat completely (You dont need it unless your a jessy in the cold weather
) - This removes one possible fault, It sounds like you may have a blockage OR airleak.
Feel all hoses once engine is 'hot' and you will be able to identify a blockage.
Airblock - Turn heaters on full whack, remove cap off headertank, locate the top radiater hose and squeeze release squeeze until you see bubbles appear in the header tank
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