Hi mate;
I concur with the guys above, likely a burping problem, as long as temp has not damaged anything.
Whenever something happens on a car and someone brings it to me, first thing I say is "what has changed??" Because "Car running fine, do something = have problem" - SOMETHING is where the likely start of it is.
I have had people say "They did xxx, but couldn't be that, because they fixed it..." But car was fine until then???
Firstly, I am old and I don't trust anything. The thermostat. Your car ran fine before, now it doesn't AFTER new thermostat. I always test "new" things. ONCE (ONCE!!!!!) I didn't test a radiator cap on an old car, meant to be 13 lb. It blew the hose off. When tested, it was 19 lb. These items are cheap, and remarkably reliable, but every now and then get a dud.....................So - I would test the thermostat. MAKE SURE it opens when it should. Just boil it on the stove with a thermometer in the water, watch when opens, make sure it opens all the way. Make sure temp isn't reading direct gas from stove etc etc (you know the drill).
Secondly, temp gauge. Good to read the EVIC temp, that should be ok. I would double check. IR temp checker is 8 dollars, or $20 if you "get a good one". Be careful as different surfaces give different readings sometimes, but take a heap of readings, can work out what is spurious.
The temp gauge in new cars often isn't linear. Otherwise people would panic. As these engines run hot; the "operating range" they are meant to run within is compressed in the middle of the gauge, and is a component of the engine monitoring "showing what people want/need to see". That is why the needle goes to about the middle, then hardly moves as temp wanders up. When the computer gets to a "warning temp"; it will then elevate the gauge, but often by then warnings (lights, even codes) will start to take over.
The point is, the gauge will sit "at normal" for a bit of a range that it deems "normal"; so if you want to see any movement, use the EVIC, (which you are, great) once you confirm that THAT is correct.
Remember that EVIC can only read correct temp if correct circulation, otherwise it will be a "hot spot". Fan belt tension, water pump (incorrect clearance, but yours new????), thermostat not opening/not opening far enough can all cause that. Again, IR will show if temp even throughout or not. Is radiator red hot when it is doing this? The average old blokes hand can tell the difference between "Hot" (normal) and "HOT hot" (overheat)..............
I would remove thermostat and test. Test the old one. See if any difference. THEN I would burp again, go from there.