Idle low to cutoff
 

I have an audi a4 2.8Q, recently when I downshift and come to a quick stop the cars idle goes very low, jumps up and down alittle and cuts off. I put fuel injector cleaner in the tank, and the car started to work better. Till finally its fine now. I'm guess the fuel filter is clogged. Im not sure tho, did anybody have a similar problem? Does anybody know where the fuel filter is located?