Engine temperature question

[Thatsmrpigg]

Hi everyone,

I am new to the forum and had a question about the engine running temperature of my 2007 ES 350 with 179k miles. 

I have noticed in the last few days that when driving at highway speeds in 65 degree weather the engine temp drops to about a 1/4 on the gauge then when I come to a stop and idle for 2 minutes or so the temp will rise back up to one tick below the 1/2 way mark on the gauge (which is where it usually stays when at operating temp). 

I have not gotten any warning lights and the motor still seems to run smooth and strong with no noticable change in idle. 

I feel like it is possibly the thermostat that is stuck open but wanted a few more opinions.

Also, how difficult is it to change the thermostat in the Lexus ES 350. I am fairly handy and do some of the work myself but I'm definitely not a master mechanic.

Any thoughts or suggestions would be greatly appreciated.