My 2009 nissan titan overheats.?

It has been hot these couple weeks over 110 regularly when i am stopped or driving slow, like when i cross the border, my truck starts to overheat. I turn the heater on and it cools, but as soon as i turn the ac back on the temperature starts rising quickly. if i turn the ac of it also helps cool, even if the heater is not on. Any suggestions I thought my fan might not be working.