I wondered if anyone could kindly help with a problem that appears primarily from a cold start with my Santa Fe (90k miles)?

Occasionally, the engine starts and ticks over slightly unevenly but there is no power available, ie putting it into drive won't get it to move up the small incline on my driveway. The throttle has no response. Black smoke pours from the exhaust and there is a smell (to me) of unburnt diesel.

Eventually, after a number of ignition restarts, the engine begins to rev as normal and I can drive away.

Recently, I made it up the drive, but it reverted to a bare tick over and limited power (as described above) which gave me maybe 3/4mph on the flat. Eventually I reached a nearby down hill, picked up speed and there was an audible metallic clang and the engine then picked up as normal on the incline.

Usually, once these cold starting problems have been overcome the engine 99% behaves faultlessly thereafter. However, very, very occasionally, when warmed up, say when the engine has been at tick over eg in a traffic jam, or when having been in foot off mode slowly approaching lights, there is a short time loss of power which recovers itself quickly with a surge (as the throttle is usually depressed).

The garage seems to think it is a glow plug issue but with the issue occurring when fully warm I am not sure and I suspect carburration.