Car stalls, shakes, doesn't start?

To start my car I have to hold my foot on the accelerator and keep it on otherwise it will stall. When at traffic lights if I take my foot off the accelerator the rpm drops really low and the car shakes and sometimes stalls, or sometimes not shake at all and stall but only when at low speeds. At higher speeds the rpm seem normal but the car shakes violently. I thought it was the tires but we had them replaced aligned and balanced and it still shakes. My car now won't turn on, it has done this before but started working again a couple of days later.

The car is a Volkswagen polo 1998 auto