2010 Kenworth W900 - Cat C15 - High Coolant Temp but Oil is Cool

I have an SDP serial C15. On flat ground, she runs at 190°F all day. As soon as I hit a grade, the coolant temp shotguns up to 220°F and the fan kicks on high and won't turn off. Surprisingly, my oil temperature doesn't move much at all, staying around 200°F. Usually, when these engines get hot, both temps go up. I've replaced the thermostats (regulators) and the water pump was done last year. Radiator looks clean from the outside. I did a pressure test on the cap and it holds 15 psi.