What do you think about this issue

[Chevy Fleetside]

I drove my truck down to San Jose to pick up a piece of funiture. I have been having trouble with it getting hot on the freeway so I thought I would change the thermostat. The one that was in there was sticky so I replaced it with a 160º thermostat and I have a feeling that is just to cold and won't close when the water is hot so the radiator can cool it down. The first picture shows what the temp is like when it is hot outside and the other is when I came back into cooler weather. The truck cools down alot. It was probably 80º in San Jose and about 60º in SF.
I have changed the radiator. I think the next thing I am gong to do is to swap out the thermostat for a 180º.