Gears stay stuck when A/C is on

I have a 2000 v6 mustang and had high idle problems but changed out the IAC, now when I drive with the A/C on the car acts as if it is in cruise control and won't downshift when braking. This happens if I step on it too fast or go past 40 MPH and I take my foot off the gas and it stays stuck on 40. Any help?
 
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