Fighting with engine temp

Have a 68 Dart GT with LA 360. I dropped from a 195 F to 180 F to 160 F thermostat. Daytime, temp gauge stays in the middle during 35-50 mph cruising, almost 3/4 on freeway driving. At night the gauge goes to 3/4 on freeway driving and cruising 35-50 mph. Then if I punch it, the needle goes down a little. She drives fine and engine sounds strong, not pinging, engine seems cool when I stopped and checked. This occured tonight. You guys think my water pump is going????? When I first got the car temp gauge would stay in the middle regardless cruising or freeway, day or night. Then maybe a month later at night it started to go to 3/4 on the temp gauge. Maybe the stock temp gauge is not reading correctly?? Iam putting in some Autometer 2 1/16 in water temp and volt meter gauges this Wednesday. Please advise.

Thanks,
Chad