2002 Chevrolet Silverado 4.3 heating problem

Hello,
I have a 2002 Chevrolet Silverado 1500 4.3 V6 Vortec that heats up when I’m in traffic and the A/C is blowing it only gets to about 3/4 of the way to the red line and stays there till I take off. I have to shut off my A/C or it will keep on heating up. I’ve checked for air pockets in cooling system, checked my thermostat, checked my fan and clutch they both kick in. My coolent level is always where it’s supposed to be never let it go empty. The only thing that I did find was a small leak in I the cooling system but it only leaks over night. When the engine is running it does not leak. Please help me to find what is causing it to heat up