The Nissan Path

At first I thought it was due to my recent radiator change resulting in low coolant levels.
Symptoms is, air flows but is cold.

But I parked the vehicle nose up a couple of times and the rad is filled to the brim.

A restart sometimes fixes this instantly.
So I am thinking a stuck HVAC door in the dash?

Any thoughts?

Did you try my purge procedure? Get the front end up as high as possible, run the engine at 2500-3000 RPM for at least 10-minutes with both front and rear heaters on the hottest temp setting, or longer until hot air (about 140 degrees F.) comes out of the heater vents? By your description, it sound like an air pocket is stuck inside the heater core.

Sir,

I will try your trick but I feel it is an actuator door of some kind.

Reason is (and I neglected to mention this last time, my bad) when the "no heat" condition occurs the air will not only be cold but also come out of the wrong vent.
For example HVAC set to "feet" and air come out of dash.

Well, those are controlled by different actuators, one for the mode (direction of airflow) and the other for the air/mix (temperature). Usually when people complain about the air coming out of the vents as being cold or not hot enough on these vehicles, it is usually due to air trapped in the heater core or a bad (stuck open) thermostat, with the majority of them being trapped air. Another thing with a lot of these vehicles (including mine) as that you always feel a little bit of cool air coming from behind the dash, around the area of the driver's right knee. I'm not sure if it's just a design of flow-though ventilation or just poor sealing on Nissan's part, but is very common.