I now know why the temperature is dropping. I'm not sure why it suddenly started but I do have an idea.
This is a small 18 minute print. I had set the temperature to 205C and 70C and when it stabilized I started the print.
1. First layer printed at 205C and 70C.
2. Temperature drops to 200C and 65C for subsequent layers.
3. After 3rd layer or under a certain level size the auto-cooling rules take effect and the external head mounted cooling fan kicks in at rates between 30% and 100%
4. Element can no longer keep temperature at 200C until I unplug the external fan.
5. Temperature remains stable for the rest of the print.
I honestly can't remember if I've seen the fan spinning in the past. With all the work rewiring the fan, heater and sensor I might well have plugged in or mechanically re-installed that fan so that it now started working.
Or, the replacement (now ball bearing) extruder fan that runs continuously is way more efficient and the airflow past the heat sink down towards the bed now cools the extruder body and top layer of the print. Add external fan and now the heating element can't keep up.
Power supply is steady at 11.99VDC +/-400mV. The variation of the supply voltage (monitored with a scope) while printing doesn't show enough of a dip or sag under load that would cause the heating element to drop.
One potential solution is to bump up the power supply by a volt or so to add some reserve capacity to the extruder heater.