Kia Cerato 2007: Engine miss and broken speedometer

I bought a 2007 Kia Cerato a few months ago. It has two things that most of the engineers I have consulted could not solve. One is an engine miss. The spark plugs were changed and a tune-up done. It was ok for two days and then it started to miss again. Then I changed the extension wire ignition coil and one ignition coil that was not firing, but it did not solve the problem. The second problem is the speedometer stops working after driving about 7-8 km. If you stop the car for two hours and start driving again it stops working in another 4-5 km. I have showed these problems to the Kia engineers, but they haven't been able to help. Could you advice me?