Intermittent heating up at idle '89 Legend

In warmer weather, and only on occaision, I have a situation where the temperature gauge will shoot up to hot in a matter of a minute or two while sitting at a stop light, etc. As soon as I start driving, the temperature gauge goes back down to normal. I have discovered that if I turn on the A/C, this helps stabilize the temperature, even at idle. I had a new thermostat installed, fans are operational. It was suggested that there is a thermosensor in the bottom of the radiator that signals a fan relay switch that could be the problem. What about the coolant temperature sending unit? Anyone else have a similar experience? It is getting costly, taking the random approach to replacing parts, hoping to resolve problems. This car is turning into a money pit!!!!

My 92' Legend does the same thing and I have replaced the thermostat,waterpump gasket,head gaskets, and flushed the radiator and block. Acted normal for seven months over winter and started up again this summer.