I had an Escalade doing this. Leak was found to be in the rear, passenger side where the heater hoses connect tot he rear AC and Heat.
With your miles I would have the radiator tested for Hydro carbons, you can also buy a kit at any local parts store to do this. If the kit finds hydrocarbons in your radiator the head gaskets are leaking, if not they are good. Quick and pretty cheap test to rule it out. I lean towards this since your idle is fluctuating, the engine is compensating for something.
With your miles I would have the radiator tested for Hydro carbons, you can also buy a kit at any local parts store to do this. If the kit finds hydrocarbons in your radiator the head gaskets are leaking, if not they are good. Quick and pretty cheap test to rule it out. I lean towards this since your idle is fluctuating, the engine is compensating for something.