ISX coolant temperature go higher the normal

[Chamberpains]

If you work in a really dirty area or a lot of miles on the radiator. It could be plugged with dirt and bugs. Visually check front and back. You should be able to see daylight thru all the condenser, CAC and Radiator.
Also what you describe, sounds like the thermostat may be starting to go bad. Mine did the same thing really slow over time and it was hard to notice till the fan was kicking on really fast going up hills and would also do it when truck was still warming up. It wouldn't hurt to change it if it's been in awhile.