Temp runs hotter on Highway

[Troy 82]

At idle or crusing around town, my temp reads just above the 1/4 mark on the gauge. At a constant speed 60 or more it goes between 1/4 & 1/2 on cool nights & just below the half mark if it's hot outside. What is the most possible cause? water pump, Radiator? I ran some radiator cleaner through it, new temp sender. Thanks!