Before you do anything, take the car to a workshop with the correct diagnostic gear and have it scanned. This process will see the workshop’s computer have a deep and meaningful conversation with the computer in your car, and the result will probably be a fault code(s) that will tell the mechanic exactly what’s going wrong.

Beyond that, you’re merely stabbing in the dark, as limp-home mode can be triggered for all sorts of reasons including dud sensors, poor wiring, a computer glitch, you name it. The fact that it’s an intermittent limp-home situation makes it even harder to diagnose without a computer scan. You cans start to replace components at random to see if they fix the problem, but you’ll probably wind up replacing a whole heap of perfectly working ones before you stumble on to the one that’s playing up. In the meantime, you’ll have spent heaps and wasted weeks.

Finding the cause of a problem can be challenging, particularly if it is intermittent, or like yours, only when the radio was turned on. You have to pity the mechanic sometimes, but it does highlight the need to be observant when you find a problem. Write down as much information as you can about what happens, when it happens and anything else that might help the mechanic diagnose it.

The car was going into limp home mode when it was losing power, that's why the speed was limited. Turning the engine off and restating has the effect of rebooting the computer. That suggests there was an electronic problem, and replacing the speed sensor would seem to be a valid course of action. I wouldn't have thought blocked injectors would cause the engine to go into limp home mode, that's a different issue. Worn injectors are a regular problem on modern common-rail diesels, whatever the brand, and replacing them is the way to go. Check your warranty documents to see if the injectors are included in the consumables list. If they are they probably won't be covered by the warranty, if they aren't you might have a claim. At best Kia might come to the party with part of the cost of replacement.