car is jerking on low rpms
 

hi.
i have a4 97 and when i'm driving on low rpms the car starts jerking and it feels like it losing power and i can't turn my ac on.
i changed spark plugs, because there was time to change it but it didn't change anything.
can anyone tell me what the problem is?
every time when something breaks in this car i am scared that it is going to empty my pocket. i am already getting tired of that. love the car but there is limit on spending $.