1999 Ford Mustang surging idle

Engine Performance problem
1999 Ford Mustang V8 Two Wheel Drive Manual 100k miles

Hello, My mustangs idle will surge to around 1200 then drop down to almost a stall while sitting at a redlight with the defrost on. Not the rear defrost but the front, with the temp all the way to the cold side. Being that I have the defrost on the cold does the ac kick on even though its on defrost and not on either max ac or ac? I changed the IAC not too long ago, roughly 1000 miles ago, then started to have this problem again so I pulled it off and cleaned it and the throttle body out and still surges. Any input will be much appreciated, if there is any other info you guys need let me know and I will be happy to provide it, thank you. Joe