2001 Kia Optima air problem

while i am driving with the air on, you will get a loud clunk, with air and lights on, vehicle will power down, lights go dim, radio shuts off. when you turn off the air, no problem. You can actually feel the clunk, it can be described as feeling like you are shifting gears as in a manual transmission. have had my local mechanic look last summer, no luck. since we have not been using the air in the winter months, all is well. now it is summer and we're back with the problem