Volvo 240 (1990) running hot - flushed coolant, but???

My 1990 Volvo 240 wagon runs a little hot when in traffic on a hot day.
It doesn't overheat, but it still makes me nervous. I flushed the
coolant, and it still does it. I also cleaned the contacts on the
temp. compensation board and installed a new thermostat. The heater
seems OK. When i turn off the heat, it stay on for a while while I am
trying to get cold air out. It is kind of a strange cooling system on
this car, there is no radiator cap, and no drain plug either - you just
take the lower radiator hose off and watch coolant go everywhere except
in the container it is supposed to. My last vehicle was a toyota truck
from the 80s and it did the same thing when I got it, but flushing it
out worked perfectly, because there was a bunch of gunk in there. The
volvo coolant seemed clean.....

Do I just need to re-flush that shizzle, or??? I have never had this
happen on any of my cars where it would not go away.