Got a 98 f-150 and it was having a overheating problem. It would occasional over heat and the heat gauge would sometimes just jump to hot in a split second.I just replaced the thermostat and sensor last night but now the gauge isn't moving from the bottom(cold) and it still seems like its overheating( u can hear the bubbling water).
Check to make sure you didn't damage any of the wiring to the temp sending unit when you replaced it. If everything is good, chances are you got a bad sensor. As far as the overheating, does it happen all the time? For example, does it overheat if you are at highway speeds or does it only happen when you are in stop and go traffic?