2008 Volkswagen Polo Backfiring and Jerking

I was driving along this morning and my car just felt like it was losing power and upon looking in the rearview mirror I saw a huge black cloud of smoke. I was driving at a speed of 100kms and as soon as it backfired it started jerking. The car has been serviced, they told me at the garage there was too much oil in the engine hence the backfiring but now the backfiring is back with a jerk. I refuse to take it back to the VW dealership as they just use the diagnostic tool and it says nothing is wrong. My fuel usage is also very high lately