mine started doing the same thing, i got a code cant remember the code right now but it was for thermostat circuit open, and it would warm up to normal temp but when cruising down the highway it would drop to where yours is in the second pic, if i was on gear and holding the gas it would be like in the first picture, but as soon as i put it on neutral to coast it would drop, i still havent replaced my thermostat because its still winter here in mn and its a $90 part.