Eureka!

I have 100% figured out that the problem is the programmable thermostat.  If I take it "off the program" and just have it set to 20C, then everything is fine.

The problem is that in program mode it seems to get stuck in "Sleep" mode and drops the house temp to 17C.

So, temporary workaround is to just have it set to 20C.

Still, I was so cold.  So I went downstairs and went for a run.  Totally warmed me up.

Break through!