1996 Nissan Maxima thermostat

Engine Cooling problem
1996 Nissan Maxima 6 cyl Front Wheel Drive Automatic

My car
19996 nissan maxima GXE automatic, has an issue with heating up properly
Although it runs fine ( no drop in performance and mileage).
The temp gauge will not reach its normal (halfway) position
Heat does not come from the car as it used to prior to this issue.
I believe it is a T-stat stuck open.
if the car sits in traffic or idles the gauge rises back to normal. Upon motion both the gauge and heat displacement drop,
There is some confusion as to the engine type,
next where is the location of the t stat.
and do I need to replace the water outlet pipe with the t stat?