Simple inspection of the rad core is to drain, look in w/ a lite, and see if the core tube ends are loaded w/ calcium/crud deposits.
If so, you can bet that the crud has collected in the lower ends of the tanks, and may have some tubes completely plugged.
As an experiment, "my 99%" 87 had the crud on the tubes. I pulled the unit, found it corroded at the tanks, and cut it open at the ends of the core. The lower tubes were totally plugged. This was on a 59K mile car. Being it is a lower mile car, the rad condition pretty much much went along w/ the pisspoor maintenance from the po's...