overheating only at highway speed

hi guys. i have a 2008 aveo 5. last fall it split a rad hose and overheated badly. replaced hose and therm.i went all winter seemed fine but as it gets warmer outside. i notice that it starts to run hot while driving on the highway. if i turn the heat on it comes back down. any ideas?

My guess would be a plugged radiator. Flushing it might help. If not, you'll likely need to have it replaced.