engine fault problem, and ideas?

Hi all, i also have an A2 1.6 FSI. my engine failure light has been coming on every few months! when it lights i set out on making the trip to Audi service. it first happened on the 30th of May 2007...again on the 7th of october 2007 and i was driving the car home yesterday(4th jan 2008) and it happened again! have booked car in on monday, more time off work. on the 30th may and 7th oct was told the problem was the pisten miss fireing, so on both occasions the coils were replaced, for a lovely sum of 170 euros about £110 pounds, the car judders, the engine sounds like a tractor! also i have noticed a loud knocking noise when i go over small potholes and over ramps..have had this looked at and was told it is nothing. it is getting to the stage that i dont know what to do. Any advice? would really like some feed back