It just started yesterday, and the again today.

1998 Dodge Ram 1500 5.7, it only overheats in the mornings, it has been cold lately, 28-32 degrees, it runs fine on the highway, a little warmer than normal, but not bad. Come off highway and the temperature really goes up. On the return trip home 50-60 degrees, still runs a little warm but does not over heat.

Its not low on fluids and I run a 50/50 mixture for radiator fluid.

New radiator six months ago, and I am changing out the thermostat today. It does not have an electric fan

My other thoughts is it could be water pump related?

Any other suggestions/thoughts?