Cab heater blowing cold air

Hi it's probably the thermostat not working properly.

If thermostat jammed open it will run cold .If temp gauge normal , check coolant level .You could check the heater hoses where they pass through the bulkhead into matrix and see if hot .Other possible causes if not any of those .

Bit of an unclear question here!
So diagnosis isn't going to be perfect.
But if I understand you the heater in the cab is fluctuating?
What does the gauge read/do,
is it a stable temp?
Is it of the low mark?
Because it sounds like air lock in the matrix,
which could be caused by a low coolant level which you topped up but did not bleed the air out.
Or the worst thing it could be is gas locked from a failing/failed head gasket.
So 1st thing would be to bleed the air from the coolant.

Didn't want to spoil your Christmas , so held back suggesting head gasket and left it under other possible causes .Bleeding cooling system is a good idea and may indicate head gasket if air found but a block test will confirm or not .