unhappy with service

I took my car to Alex motors as it was losing power whilst driving it,especialy long distance.I was refered by one of the Wolfburg mechanics as they said this is an electrically related fault.I did take it in to Alex motors,and They diagnosed it using their machine,saying it shows a misfier from cylinders.They end up saying the problem is a coil.they ordered n fitted the coil.I was charged R4280.I took the car home in Mdantsane,the following day i went to Butterworth,my work place.It started doing tge same thing i took it in for.I cakled tgem n told them that they didnt fix the problem coz ts doing the same thing. All this time i was communicating with tge resmceptionist by the name if Nicole. They said i must bring it in for durther diagnosis,but still not much progress.this time,Dylan changed the plug, but still the problem was still there.I tiok it in the third time n ts been with them for three weeks and still no progress.Until i was told that i need to buy a knock sensor.That is where my problem is,bcoz now i have to buy another part,that i think shldnt be the case,coz to me theyve bn misfixing and ive paid for nothing.I think i shld be refunded instead.