got a 2001 Dakota 4.7 a few months ago on a 10 hr road trip the truck started to get quite hot so i pulled over and let it cool for probably another 2 hrs till i could get enough service to call a tow truck, anyway when it got to the garage it was quite low on coolant so they topped it off did a pressure test and recommend a new radiator cap. so i put a new cap on it and it was fine for another month or so. then once it started getting cold out i noticed i had no heat when it was idling so i checked and it was low on coolant again never overheated but the overflow tank was overflowing. so i figured the thermostat was bad so i replaced that and put new radiator hoses and another new cap at the same time. was good for about a week and now there another issue. i still wont have heat when idling unless it give it gas so i figured the heater core was plugged because it seemed there was not any flow blew it out and there was no blockage it was low again so i topped it off again. no white smoke no coolant in the oil and no visible leaks on the ground or anywhere else. every time it was low i bled the system out and everything was filled to the correct level truck has 184xxx on it no clue what could be the issue

 

Sounds like you have a bad/leaking water pump.