Jumping needle

I have a 2011 Dodge Nitro with 78k miles. Since Friday my
thermostat needle has been going from stable (middle
line) up to the 3/4 line BUT never over that to the H line.
There is a full compartment of coolant in the car. The
thermostat, radiator cap, radiator, hose have all been
replaced with the last 5 month!! But when I'm driving the
needle is always stable on the highway, however while
driving on the street, or sitting at a stop light or sitting
slightly down hill or in traffic (slowly moving like 8am
traffic) the needle goes up and down continuously. I have
no idea what is wrong. Express oil change did a
diagnostic test an found nothing other than me needing
spark plugs. I am constantly stressing because I have no
idea what's wrong. Can anyone give me some insight as
to what they believe is wrong???