You will most likely see lower/steadier coolant temps in hot weather with the efans running constantly (a/c on). I also use a ScanGauge II to monitor coolant temp exiting the motor, coolant temp exiting the cold radiator tank and tranny fluid temp.I just sent my ob2 scanguage in to have a software update and it won't be back till next week. I had the same drop as you (200+ to 175) by putting the car heater on full blast. I didn't know then that the defroster would also turn on both radiator fans too. Did not want to use the air conditioner on a hot day going up a hill just to turn the fans on. Just found out a couple days ago about the defroster. Plus you have the heater fan on, also pulling heat away from the engine...
I will experiment more next week when I get my scanguage back.