Overcooling?
 

2001 Jeep XJ, 3.5" lift w/ 31" tires, aftermarket aluminium radiator, also have hood vents.

I have having problems with my Jeep not getting up to temperature. If I let it idle for awhile (30+ mins) it almost gets to 210, but then drops back down to between the second and third tick marks when I start to drive it, which obviously is running much to cool. The heater is suffering due to the engine not heating up, so my heat is mildly hot at best (but it is better at idle when the engine is almost to 210). I bought this XJ with a cracked head, and replaced it with a new TUPY head.

I have:

Replaced T-stat with a 205, which improved the running temperature only marginally
Replaced water pump, t stat housing, and radiator hoses
Triple checked coolant levels
Replaced head
Replaced fan clutch with a new one

Have you used an IR thermometer to measure the degrees at the thermostat housing? I have a similar situation, and my thermostat housing is right at around 210. Which is pointing towards the Coolant Temperature Sensor being slightly off.

My 95 did that with a mechanical fan clutch that had issues uncoupling.

Check the thermostat by putting in a pot of cold water let it boil and make sure it's opening when it's supposed to and not sticking as the water cools. If the clutch for the fan is questionable replace it and make sure the shroud is intact, my missing shroud seemed to make the Jeep run 10-20 F cooler oddly enough.

This problem pops up on this board at least a few times a year. You don't mention what thermostat you put in there other than the temp rating, but if it's not Mopar get it out and put an OEM one in. kbeam418 has good advice if it's already OEM, and you want to check it. Thermostats are designed to fail in the open position 9 times out of 10, and your thermostat would not be the first to do so right out of the box.

im goin to bet your heat issue is not engine temp but more likely a clogged heater core or faulty coolant valve