poor quality ngk plugs

i purchase a set of NGK spark plugs for Goldwagen Midrand <br> on 28/01/2015 price R228 invoice ***2<br> after fitting the plugs i found that my car was back firing and idle was off we drove the car for 2km still the same problem.I removed the new one and replace the old one and it work fine.<br> We found the sparks plugs supp**** my goldwagen was defected (ngk sparks plugs)<br> I than went back to goldwagen to show them the plugs supp**** was damaged and not gap right this affecting my car then decided to send the plug for warranty Goldwagen return saying nothing wrong with plug saying it was fitted incorrectly. i don't think so because a vw mac change it for me and being doing this for 20 years<br> I think that the plug sold are defected by goldwagen that ngk plugs and the poor service that goldwagen gives as they are selling that product. I think we VW drives should not by ngk plug as it can damaged your car and spend thousands of rand's to fix<br>