Ac and low idle problem
 

HELP!!! Hey guys I'm new here, I have a question I just got a 99 gt 5 speed. It idles and runs fine but it goes crazy when the ac is on, drops down to around 300 rpm and goes up to around 1500 and back around 750 if it doesn't stall. Just changed the aic today and still does it, car has bbk 70 mm TB, k@n air filter .... any ideas from y'all ? THANKS!