'85 4kq - overheating - clogged radiator?

[Ben Swann]

I have been troubleshooting an overheating problem in the '85 4kq -
formerly rally car now being pressed into service as my daily
driver/beater.  The fan switch circuitry works OK as tested from the switch
at the bottom of the radiator.  I replaced the thermostat, thinking that it
was not opening - the one I removed was definately not opening all the way.
  I still had the problem.  Car heats up at idle to the point where
pressure forces coolant out the overflow cap.  Replaced cap.    I just
installed a new thermo-switch.  It seems that the top half of the radiator
is getting very hot, but the bottom half is only warm - not enough to heat
the fan switch(also cool).

Is it possible the radiator is so clogged that there is no circulation
getting to the bottom?  What else can be wrong.

BTW I also flushed the system when I replaced the thermostat.  As a side
note, if I run the heater and vent fan on high, I can keep the car from
overheating (that was after I cleaned the mouse nest out of the squirrel
cage - another saga).

Ben