I have had that problem since day one on my 2005 Beaver Monterey 400 HP.
I talked to Cat in Ore. I was told keep the RPMS up, manual shift transmission if necessary.
I then talked to Tom @ Massey Diesel,he says the same thing, keep rpms up around 2000 or higher especially climbing hills
Makes sense, listen to the long haul trucks, they keep the eng RPM high, you can hear it.
I tried it, it works!
I am adding an Electric fan, this would only work when the Dash AC is running, Just get more air across the Radiator.