My Car Overheats When A/C Is On?

I Have a 1997 Ford Mustang 3.8 V6. My problem is my car overheats when ac is on, ac works good ice cold but on very hot day it over heats even while doing 70 on highway its just too hot. i live in ok and is 103+. i need some help running through and seeing what the problem, and its not the thermostat i replace that the other day thinking it might help but no itdidn'tt. which means i also flushed out the radiator several times, and the fan comes on when the ac is on i checked that too it comes on soon as i turn on ac. idon'tt know what else to do.i also blasted radiator/condenser with high power water atcar washh and still same effect works for a while then starts to overheat. on cooler day around 90 or so it works going 50+ and stays normal but lately its been 103+ and even 65 on the highway still wont keep it cool. open to anything please!!