2015 international prostar still overheating after new thermostat

Hey y'all so we have a 2015 international prostar with a Cummins isx. It has been overheating. We have replaced thermostat, hoses, water pump, and had it flushed. Starting to get on my last nerve. Because everybody that works on it believes it's a different thing each time. Anyways first thing that happens is the engine temp goes up then the water temp climbs extremely fast to the point the fan don't even help. It seems like it's vacuumed sealed somewhere. I always check the reservoir to make sure it is at level. Sometimes it is gone until I try to unscrew the cat to put more in then it feels back up, And other times it has a lot more coolant in there than what was originally started with. Please help any suggestions would be awesome.