Forgive me for being the bearer of bad news but... I think your car's engine is on its death bed, prepping for its funeral!
The last ride!
The thick black smoke you're witnessing is due the leakage of oil in the combustion chamber. And since oil isn't supposed to burn like petrol; the engine chokes when that happens due to lack of oxygen (Just like humans!) and inefficient combustion and hence you experience fluctuating RPMs.
I'm not new to this forum; and almost all car owners with engine problems state that the first thing their mechanics did is change the fuel injectors which is bullsh!t. Only dishonest, fraud mechanics do that.
Yes, you've been conned by a thief, my friend!
A new fuel pump or injector nozzles is going to do squat if the oil gasket is leaking, which is apparent from the thick black smoke and dropped RPMs.
Tell you what, take out the spark plugs and if they're covered in grease like thick black goo (And I believe they will be) then it's definitely a gasket related problem, not a fuel related.
And also check the compression of all cylinders while you're at it. Oil gaskets are built to last a long time; so unless your car has been sitting idly for years; I think you might also have a blown head gasket, a damaged cylinder head, a cracked engine block or worn-out pistons and hence, a top-end rebuild might very well be on its way, given the age of your car...
Hope it helps.
