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Blend door issues are the MOST common cause of your issue, IF you can confirm that the door is in the proper position when you don't get proper heat the next check would be to backflush the heater core to remove any "crud" that may be intermittently blocking it.
Checking for decent flow when you do it....
A cheap infrared thermometer would help confirm the heating issue quickly, failing that I've had good luck checking A/C operation by just putting a small kitchen (cooking) thermometer in a dash vent. That would give you some numbers to work from that would confirm things better than how it "feels", since varying outside temps have a LARGE effect on subjective estimates of temperature....
Luck!
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