1985 GMC S15 2.8v6

I needed a new water pump(dripping from hole) so I also replaced the thermostat and radiator cap and flushed the system. Now at hwy speeds it overheats but when I let off throttle the temp. goes down to normal. Also at idle there is no overheating, only at higher rpm’s. Even in very cold weather the overheating occurs at 60-70mph but if I let off throttle and coast the temp drops right back to normal in a few seconds. The temp goes right to red under load after the thermostat opens, and if I stop the temp goes to normal as it should always be. I’ve owned the truck 15 years and never had any cooling problems before. I also do all of my own repairs and feel confident in my installation process.  Could this be a fan clutch causing my problem? Any info would be appreciated.