Impossible Engin Start-up and sometimes the engine dies when driving + Black smokes -not always-

Hi Gents;

Recently i started experiencing a set of problems with my 2005 KIA Sorento, first it was hardly strating-up, it takes many seconds; after that there were some black smokes when it starts-up and some times when on 1st gear even 2nd.

but now after few weeks of this life, i started having issues with my car when driving, the engin lose power then it dies suddenly!! when i try to re-start it up...it just don't!!

when i went to the mecanics garage, they told me that it's an injector problem, i changed them -the 4 of them- not just this, i changed the HP Mano and air-filter and so many other staff!!!!!! but always having the same problem; my car always still have issues of starting-up and it does with anti-insect blow right in the air-filter "trough the engin side".

i really don't know what to do!! i am really confused, and i really want some help over here!! so if there any suggestions, please don't hesitate, and if any other info is required, please feel free to contact me.

Thanks in advance and best regards.

Abdelhak.