The Hyundai ix35 diesel I bought new in 2013 has an intermittent problem where it loses acceleration and will only slowly accelerate from 40 to 60 km/hr. Because I couldn't reproduce it all the time the dealer told me that it was the diesel fuel that had gone off, but then it happened several times again over the next few months before disappearing again. It happened a couple of times on the freeway during busy traffic and I felt in danger of other cars crashing into me, and had to pull over to the service lane until traffic cleared. In January, I drove the car directly to the dealer when the problem occurred again and the supervisor agreed there was a problem and kept the car for almost a week to carry out tests as well as taking it for drives to carry out further tests. The mechanics found a loose connection and I was advised that this was the cause, but it came back a couple of weeks later. Are you or any of your readers who own an ix35 diesel aware of such a problem? A year has almost passed since this problem first occurred, and I would like to have this problem fixed once and for all so that I can use the car for the purpose I bought it for.
I don't blame you; the problem should have been fixed a long time ago. It sounds to me like it could be related to the diesel particulate filter regeneration, but whatever it needs a proper investigation. Take it back to your dealer and ask them what they plan to do about it. We will contact Hyundai on your behalf and request their assistance in fixing the problem.