I would really like your opinion please.

Hello, it's possible that a spark plug could be improperly gapped, or could also have inadvertently gotten cracked upon installation. I'd remove the spark plugs of any affected cylinders and inspect them closely for any signs of cracking and if good, check the gap. Make sure you only use original Hyundai plugs and coils for best performance. At times, certain aftermarket components have been found to be less than capable for certain applications; resulting in -- yes -- check engine lights and misfire codes after replacement. Whether or not any of these repairs will affect the performance issues you were having is unknown. It may be that you need a professional inspection of the engine performance by a shop near you. www.repairpal.com/repair-shops