What might be the problem with my 2000 Grand Cherokee 4.7L if the temp starts to rise after 10 minutes of driving but not if the truck is in park yet the belt water pump fan clutch and thermostat-new?

The problem might be air. If an air bubble is trapped in the cooling system, coolant is not circulating as it should, since a portion of the closed system is occupied by that air. As a result, what should be a column of coolant is not reaching the coolant temperature sensor. What reaches that sensor instead is hot foam, which does not have the capacity to transmit heat to the sensing portion of the sensor as coolant does. WHAT REACHES THE DASH GAUGE, THEREFORE, IS NOT AN ACCURATE REPRESENTATION OF THE TRUE ENGINE TEMPERATURE. While at idle, the engine is not circulating coolant at the same rate as it circulates at road speeds; therefore, when the vehicle is traveling down the road, more coolant reaches the sensor, and a reading which more accurately depicts actual engine temperature is rendered at the gauge. This is a condition which is harmful to the engine, and is easily cured. It is harmful due to the fact that the air involved routinely finds its way to the uppermost regions of the engine--- most notably the cylinder head--- which heats excessively due to the lack of cooling. The result is a blown head gasket, or a warped head, or a cracked head or block, or any combination of these. At the most severe point, this can lead to the need to replace the engine. It is easily cured through the process of burping and bleeding the cooling system. This involves topping off the coolant on a cold engine, opening a line leading to the heater core, letting the air out of the line, reattaching the line, and starting the engine with the radiator cap removed. The engine should be run until the thermostat begins to open, at which point the cooling system will begin the exchange of coolant between the engine and the radiator. NOTE: THE COOLANT ARRIVING FROM THE ENGINE IS HOT, AND CAN SCALD. CARE SHOULD BE TAKEN TO STAND BY WITH THE CAP TO THE RADIATOR IN ORDER TO INSTALL IT BACK ONTO THE RADIATOR SHOULD THE FLOW BECOME UNMANAGEABLE. THE OBJECT IS TO ALLOW AIR OUT OF THE SYSTEM WHILE THE SYSTEM IS TOTALLY OPEN AND CIRCULATING, AND THIS IS ACCOMPLISHED WHEN THE THERMOSTAT IS OPEN AND THE ENGINE IS RUNNING WITH THE CAP REMOVED FROM THE RADIATOR. Once coolant begins to be drawn from the radiator into the engine by the water pump, the level in the radiator will drop. IT IS AT THIS POINT THAT COOLANT IS ADDED TO THE RADIATOR AND THE THROTTLE IS OPENED WIDER(ACCELERATING THE ENGINE TO A HIGHER RPM) TO FORCE AIR OUT AS IT IS FORCIBLY DISPLACED BY THE TURBULENCE OF INRUSHING COOLANT. Once the radiator level peaks and will hold no more, the cap is installed, and the throttle is allowed to return to its normal position. The next step is to turn the heater on HIGH HEAT, with the fan running, and cycle the accelerator through a repetition of high-low RPM pulses to displace the last bits of air toward the radiator. When full heat is experienced through the heater, the engine is shut down and allowed to "heat soak" until cool. Once cool, the radiator cap is again removed, and the radiator is topped-off. The cap is then replaced. Topping off the recovery tank and replacing its cap completes the process. The cooling system has now been burped and bled, ensuring a solid column of coolant in toto. Happy Motoring...