Q7 3.0 tdi issue

[Borat]

Once car has warmed up and temp guage is at 90 degree and i am driving it stays there. If i go and put aircon on say on motorway the temp drops to the notch below which i think is like 70 degree. If i carry on and turn off aircon it goes back to 90 degree. I am thinking the aircon fan is cooling the car too much and its not compensating for this? Anyone know what the issue is would it be thermostat or something else. It in traffic with aircon on or off just seems lumpy like its getting hot but not going over 90 degree then.

[Steve Q]

Could be a thermostat. I'd recommend a diagnostic check to see if any fault codes flag up