A4 problem

Hello!

I would like to apologize for all mistakes that ill make in this post since im not from english speaking country.

I recently bought Audi A4 '95 1.8 turbo, and when I was testing it i noticed that when I press gas pedal to full at low RPM it feels like its working on 3 cylinders and it chokes for a second or so. But the seller convinced me that it is hapening because of chip tunning and that at such low rpm engine cannot consume that amount of fuel at once. It was really not annoying since it was really only hapening at low rpm with pedal on the flor which i never do in usual driving, so i thought its no problem. I bought the car and I made 4000km and the situation has worsen. Now even at 4000rpm if i put pedal to the flor it wont produce high power instead it goes like one or two spark plugs arent producing sparks. I realy cannot express my technical thoughts in english I hope u understand me. Its like engine is choking. Now i have low experience in cars but i have a list of things in my head that might cause the prob:

fuel pump failure
chip
spark plugs or cables

When i drive normally nothing is wrong. Please help me with your advices what could be wrong and if someone had this kind of problem please tell me what can i expect before i go to car service.

Thank you all!
Bye