'96 Ram 1500 w/360 Eng. OVERHEATS slowly?

The hotter the weather, the hotter it gets! Doesn't matter if it's at idle, or going down the road. The person I bought it from said he had just replaced the fan clutch, and thermostat. When driving and heating up I can see by the gauge when the thermostat opens, but after that it steadily climbs until about 240 or so and then levels out. There is no coolant loss! There is no loss of power, or pinging! I pulled a boat about 250 miles like this w/no problems! Could this be a faulty sensor or gauge? Or would it be something more like the radiator?