Toyota Repair: 86 Cressida air conditioning causes overheating, radiator fluid, wire connector

Question
On a very hot day, my 86 cressida (80000 miles)started to overheat after the air had been running on high during a stop and go shopping trip. On normal (85 degree) days it's fine.  We had the system pressure checked, the radiator fluid changed (really needed it)and a new fan installed. Now, when I start the car, the gauge jumps around for a few minutes before settling in the middle.  Am I really overheating, or is my gauge broken?

Answer
Check the electrical connector going to the water temp sensor, it might just be a loose connection, if the needle moves rapidly it's an electrical issue, you may want to just replace the sensor and see if it helps, it's located on the water outlet housing and has a single wire connector.