09 Prostar running hot

[Mt farmer]

Hello! New to the forum. I have an 09 Prostar, with a isx 450. Bought the truck used from Penske with 250000 miles. Stock radiator pluged too easily o I put a feedlot radiator on it and solved my problem for awhile. Truck sat for 2 months after harvest. Started hauling wheat, and the truck started running hot, like 225 on the hills. Took it to the local Cummings shop, and after 1500$, they could find nothing wrong. Radiator is clean, water pressure is normal. No codes. No leaks, not using coolant or oil. Truck will heat over 200 on high idle. Fan is constantly kicking on, so no problem there. Did the egr tune up, thanks to this site! But still running hot. Also put in a new thermostat, with no luck. Should I just throw a water pump on it? Have run out of ideas. Has the original egr cooler.