Withamxtrype., what you are probably experiencing is that your new thermostat is allowing too much coolant to flow through it and this is causing too much coolant to flow to the radiator and it is getting overly cooled. So now, when it makes it back to hte engine, while the engine thermostat is attempting to control the temp, it will still allow a minimal amount of flow to occur and this is enough to drive the engine temp down.

 

 

Withamxtype: I drove a car (not my X-Type) thru a winter storm with a broken thermostat: the engine and passenger compartment never warmed up. Quick fix: covered ~1/2 the radiator with a paper map (back when we used paper maps) and adjusted it to warm the engine without overheating.

In the northern US & Canada, diesel trucks often have a grill cover during the winter for this purpose. Heat loss is reduced by restricting coolant flow with a thermostat, and / or restricting air flow with a radiator cover.

If I understand your post correctly, your engine temperature drops when you turn on the AC (causes the radiator fans to spin faster), when you drive around (pushes more air thru the radiator) and when you turn on the passenger compartment blower (pushes air thru the heater core). If reducing airflow thru your radiator causes the engine to warm up, Thermo's diagnosis is confirmed - your thermostat is allowing too much coolant flow thru the radiator.

Of course, take care not to overheat the engine by restricting the air flow too much, or too long.