Deleted Coolant temperature switch (A/C) = issues?

[scheides]

A little history here, I have a 2G with a 6-bolt swap with a big FMIC and aluminum radiator that has overheating issues *ONLY* when the a/c is running for 10-15 minutes or more. Thermostat and all lines replaced last year.

Lately I noticed that when I have been accellerating/decelerating in traffic (full boost, then let off, stay in gear, then boost some more) my temp gauge drops way down and then back up to normal, as though there is an electrical short or something. The coolant is not low (making the sensor pick up air or anything), but there is a bit of oil residue on the sensors.

Now, I went to investigate this gauge-blipping issue today and I noticed that of the three tapped holes on/near my thermostat housing, two are present and connected (noted as labeled in the 1G haynes manual):
-Coolant temperature sending unit
-Coolant temperature sensor (ECM)

But there is simply a plug where the third unit should be:
-Coolant temperature switch (A/C)

So, what is this missing switch for and does "(A/C)" have something to do with air conditioning or some other term I'm failing to connect here? And because it is deleted, could it be causing these overheating issues? I can take pics if needed, but the sensor/switch that is missing is the one that threads directly into the thermostat housing, while the others thread in just below it.