A/C blows warm air.

1999 Nissan Sentra

i am wondering why the aircon of my car nissan sentra 1999, when you select 1 on the aircon dial, only a little amount of air blows from the aircon, but if you select no. 2 on the dial, large amount of air blows. Just two months ago, when you select no. 1 on the dial, the inside of the car is already cool, but now when select no. 1, little air blows from the aircon. Can you help me with this problem? Thank you very much.

When I drive my vehicle with the air conditioner on, when I stop or downshift to slow down my vehicle jerks and the vehicle turns off. When the rpms go down or when I stop the vehicle turns off, with the 105 degreee weather I need my air conditioner. What is causing this problem, I dont have a mechanic and I want to take it but would like to know something before I find a mechanic.