04 6.0 2500 running hot

[Low Elco]

My neighbor has a 04 2500 with a 6.0 that's been running hot. He had a friend put a thermostat in it, and they airlocked it. The consensus was that it needed a waterpump. I did that last night, and added the better part of 2 gallons more antifreeze. It fired right up, heater got hot, all good, I put the cap on, let it sit idling for 10 min while I tidied up, it was at around 190. I took off and within a mile I was at 240. I shut it off and coasted back to the house, and it was back to 220. I fired it up and it was steady at 220. Any one got any ideas? Thanks, Chip.