Shakes or Wobbles problem
2002 SEAT Ibiza Two Wheel Drive Manual 71000 miles

I was recently driving when i noticed my car was loosing power and started shunting as if the car was mis firing. after about 25 mins the a lot of smoke came out the exhaust and there was a screeching sound under the when i put my foot on the accelerator. i took the car straight to the garage who plugged in the diagnosis machine and could not find anything wrong, i consequently drove to another garage (my car still shunting and low in power) who again couldnt diagnose anything and said that they drove the car and it felt fine, however only drove it a short distance. there is obviously something wrong?! i also filled the car up with unleaded petrol before this happened. Could there be something wrong with the fuel? (i did put correct fuel in the car).
any ideas?

Thanks