Getting hot on the highway with the AC on.

Your radiator might also be plugged up with calcium and lime deposits. I have cleaned out many rads with just white vinegar. Take the rad out drain it good lay it flat on the driveway with the hose ends up, put the cap on and fill it with white vinegar, usually 1 gal will do it. leave it for 2 - 3 hrs drain and flush with garden hose for a while to get everything out. you wont be leave what comes out.