Engine overheating, Cab underheating

In Alaska, wintertime, often below freezing.  Using the heater a lot.  

For a short time now and only when the truck is idle, and then only rarely, the
temp from the vents will drop.  Days will pass without incident.   And when
the vents have dropped it's always occurred when the engine is idle at 800-
1000rpms (eg. at a red light, arriving at a parking lot, coasting downhill) after
driving, and always the vents return to normal function when the RPMs
increase. During this short time when the vents occasionally do this, the
engine temp has never wavered from the norm.  

Yesterday the heat in the cab dropped after about 10 minutes of running.  I
was idle - in park - but now the engine temp rose.  That's new.  I turned off
the truck.  The temp did not rise to dangerous levels.  I gave the truck the
usual time to cool and went to this forum.

The first thing I tried was replacing the heater control valve (a 2 valve or 2
port thing, I don't know what you call them. There are two hose connectors,
and the local maintenance shop matched me up).   A quick fix that seemed
like it'd be the culprit.  It wasn't - and yes, the vacuum control line is
reattached.  I felt the hoses on either side of the heater control valve - yes,
they're warm.  

So one of the forums here had these instructions (see photo), which I
followed. Except when I got to the big, oversized, upper radiator hose - mine
never got hot.  The coolant reservoir is mid-full; the radiator is topped; the
seal on the radiator cap applies correctly and tight.  So now I'm thinking a
blocked...something - "thermostat"? But I don't know.  I think I see the
thermostat in the engine.  Could it be air in the line too?  I don't know.  I don't
know how to diagnose this thing.  

So here's what I experience.  Whenever I turn on the truck it idles and
accelerates and revs normally and smoothly.  I can drive it just fine, and the
engine temp stays in the middle.  There's always, initially vent heat
whenever the engine is warm enough to put out heat, as is the norm with
any vehicle when first started.  When the vent cools it always happens
around ten minutes running time AND is activated only when the RPMs slow
to idle (red light, parking, coasting).  Whats new is now the engine temp
needle reflects this vent temp drop by increasing beyond the middle.  And
what is also new is that re-revving the engine or increasing the RPMs (eg.
driving) doesn't return the vents to normal operation.

I'm unwilling to test further when I see the engine temp needle move a
smidgen upward cause that's playing with fire.  So when one rev doesn't
return the vents to normal operation, I turn off the truck.  When I restart the
truck after a few minutes (not enough time to totally cool by any means) I
have vent heat immediately and the engine temp is normal.  It's weird.   
Combine that with no heat in the upper radiator hose at any time, and a good
heater control valve...

...and what do I got?

Thank you.