Why is my truck overheating only on highway?

What is your question?

New thermostat, not leaking fluids, has antifreeze. Would a new radiator fix this? Doesn't overheat in stop and go traffic, only on highway, must stop to let cool.

What makes this problem better or worse?

Stopping to cool off

How long has your 2001 Mazda B3000 had this problem?

couple weeks just bought vehicle