nyc09

i noticed all year till this moment , my temp dial on the dash has been working fine. and i notice the heat and cooling have been fine.
but last winter and also this approaching winter, once the temp starts to dip.
it takes longer for the car to warm up and show on the temp dial.

usually during the spring and summer, once my car starts up the temp dial starts increasing gradually and reaches it's peak once i start driving, usually 2/4 or so.

now that the temp is dipping, the temp dial slowly goes up and stops just a little past the C mark( cold mark) , once i start driving it barely or rarely increases and i get sub-par hating when i turn on the heat, it blows lukewarm and sometimes wamr. then it goes back to "c" dead cold.
i have replaced the thermostat a couple times and i'm positive it's not the thermostat.
anyone have an idea what's causing this issue? do i need an engine flush, radiator flush,

i heard it might be the temp sensor, but i doubt it, i say this because if the heating has been fine all year long till now, it can't be the issue.

also it can't be the heating rods either, because when i last checked the heat this early fall. it was baking when i turned on the heat except for this current period where the temp starts dipping.

any suggestions/ solutions may be helpful.
i've been to a few mechanics and they all say diff things.- heating rod, thermostat e.t.c

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