Why is car jumping and losing power?

Does anyone know why my ford ka 2000 W Reg would be jumping and feeling like its losing power while driving?
It all started few weeks ago when I unknowingly filled up my tank at a fuel station with dirty fuel only found out later this information. The car started to jump a bit so I took it to garage and my fuel filter was changed as it was clogged I also put reddex in the car hoping this would clear out the system.
Once I had been to garage the car seemed to be perfect running amazing but now the jumping has started again and the feeling of losing power but 10 times worse. Went back to garage and mechanic did a full check and even put car through diagnostics test but said he couldnt find anything wrong and that my is mechanically sound. This is still happening and even near caused an accident yesterday on a national speed limit road the car just decided to start jumping and lost a bit of power causing me to slow down and the car behind me nearly hitting me.