The problem is more than likely the heater matrix blocked up. You could try Rad Flush but the heater matrix will probably need replacement.
Also check the condition of the radiator that it's not corroded on the fins or even the radiator blocked or even an air lock in the system.
What your looking for is either bad circulation in the cooling system.
Trouble is the problem is more evident at this time of the year because this is when the heater is used more.
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