Overheting at idle

My 1991 Toyota Cressida recently try’s to overheat if I am stopped for 10 minutes or sometimes longer. I first noticed it when I was stopped at a fast food drive through. As far as I know, it has not got so hot as to boil over. The highest temperature I have seen on the gauge is slightly above 3/4, it generally runs at 1/2 or less indicated on the gauge.

If I increase the engine RPM while stopped or drive the car to increase the air flow over the radiator the temperature goes down. To date, I have replaced the following components trying to resolve the problem: radiator, radiator cap, thermostat, water pump, fan clutch and belt.

The car is not using any water and I did not notice any evidence of water burning on a spark plug when I tuned it up last.

What should I do next to troubleshoot the problem? Is it possible I have a small leak in a head gasket or cracked head. If so, what procedure would I follow to trouble shoot before I spring for an expensive shop repair.