First possiblity:
But there is this persistent problem (Three Years Now)
Rear heater core has been blocked off.
Obvious question :
Has the rear heater been disconnected for the same amount of time of your issue?.
If so reconnect it.
Second possibility:
Considering you state you’ve replaced MOST of the components in your cooling system ( you never mentioned the cap or hoses )
I might suggest you try the cap if you haven’t replaced it already.
Also check for a hose(s) collapsing causing a flow restriction.
Third possibility:
You may have a partial coolant blockage inside the engine block.