Why would the water temperature on a 1998 Ram 1500 5.9 be spiking irregularly around 240-250 if turning the AC off drops the temperature some?

Answer 1

Possibly a plugged (internally or externally) radiator. your A/C condensor is located in front of your radiator, and generates alot of heat of its own, when you turn on the A/C, your putting that heat directly through the cooling system. the radiator and coolant cant circulate fast enough to cool properly. when you turn off the A/C, incomming air flow cools just enough to bring your water temp down.