I recently installed a Hunter 44110 digital thermostat that replaced a Honeywell that experienced the same problem. The problem is that the thermometer is reading too high of a temperature (it'll read 84F when in fact it's 72F) when the AC is on. If I switch it off and let it rest, the reading will go down to the correct temperature, but as soon as the air conditioning turns back on, the internal thermometer spikes again.

This is in a mobile home with a new air conditioning/heat pump system and the thermostat is almost directly in line with a vent, so if the air or heat is on it's going to reach that thermostat quickly.

What could be the causes, and possible solutions?

Thanks in advance.

How do you know what temp it is supposed to be if you have cold air blowing on it? Shut down the air vent or block it off.