Bought this car few weeks back. Was told that it was running rich and blew black smoke from exhaust accasionally. Previous owner said he changed air flow meter, changed leads radiator n spark plugs but still problem persist.

So once we had it w e noticed it idles when u have stopped at lights or intersection revs up then down and some times cuts out. Also when driving it jerks. Since having it we have checked all hoses and tightened clamps changed spark plugs changed boost solanoid. Hasnt changed still javing issues but hardly any smoke coming from exhaust. Any suggestions on what the problem could be or what I should do/check/replace?