I would put a new thermostat in it. The cooling system is so large on these trucks that the thermostat rarely goes full open, so they develop wear spots that take more heat to crack it open. What you are describing can occur on an old thermostat.
Go genuine Cummins, from Cummins and NOT Dodge.
For reference the 190° thermostat is what your truck uses. It cracks at 190° and goes full open at 207°, max allowed is 225°. If your pulling a big load up a big hill on a hot day 215° isn't a big deal. I find it hard to break 215°, but getting there occurs a few times a year when towing.
