Combines 2166 high temp warning light

Sounds like the sensor for the light needs to be replaced.

This probably sounds stupid, but depending on where that sensor is picking up that reading this could happen when the waterflow is reduced by idling the engine. For instance your gauges pickup could be in a location that is more average in temp and representative of the overall system. Your sensor for the light could be in a volatile or dynamic location where heat changes quickly. The reduced flow from idling the engine could allow temperatures to "spike" in certain locations for a few seconds until the systems re-averages itself out.

Thanks to all for your suggestions. Sensor is behind altenator. The wire to it appeared to be partly frayed at the crimped connector due to not enough relief in the wiring harness. I rerouted the wire and put a new connector on. Seems to have helped. Thanks again.