1990 240 wagon running hot??

Hello, my 1990 240 DL wagon's temperature gauge gets up near the red
zone after idleing on a hot day for 20 minutes or so. I have flushed
out the coolant, and after suspecting there was an air bubble in there,
tried to "burp" it through the upper radiator hose, but it was
completely full of coolant.

I should say, I am using regular green coolant and not the Volvo stuff,
because I have a hard time believing there is a difference. Also, I
have no idea if it is original, but the radiator is the Volvo plastic
one. There are only a few things I can think of.

One being the radiator has accumulated gunk or rust along most of the
fins, and is not cooling the coolant properly.

Two, the fan clutch needs to be replaced, but I just swapped mine out
with a different used one, both of which seemed OK (did the "turn off
the car- see if it turns after engine stops" test).

Three - its the temperature compensation board - although the times
that it gets hot make sense because it's always after driving and then
idling for a while in 95 degree weather - although the inside if the
dash could be hot, too, and causing the temp. comp. board to
malfunction. It goes up steadily and slowly, and then I usually shut
off the vehicle, because I'm scared of it overheating. When I start
the vehicle again a few minuted later, it has usually cooled off a
little, but goes right back up close to where it was near the top red
area.

If I go look under the hood while it says it's so hot, though, nothing
feel hotter than normal to my hands. the expansion tank is not
leaking, nor is anything else that I can see. The air blowing into the
cabin gets a little warmer (I think), however. It hasn't happened when
the AC is on, but possibly always when the dash is blowing air at me.
The blower doesn't sound all that great, but, could it be causing
this??