power loss and surge and rpm loss and pick up

after driving car a while in high heat with A/C on, car will lose rpm's and feel like it's going to stall. then when you apply gas it will then catch, rpm's will rise and then power will come back after a second or so then rpm's will drop again and no power and then all of the sudden the power and rpm's will return.. surging feeling while this is happening and rough idle. once car cools off no problems again. seems to be heat related when outside temperatures get above 89 to 90 degrees... and while running A/C car didn't stall out, but felt like it was about to. didn't matter if on level ground or going up hill or down hill... car has over half tank of fuel.