fiat stilo jtd fault

Hi, i have a 2002 fiat stilo 1.9 jtd 115 bhp, it has developed a fault - sudden lack of power when driving ie ne response when the acceleraor is
depressed. however if you depress the clutch then rev it, it blows out black smoke then seems to clear its itself and will run fine. I have recently had it serviced including filter changes and a new air flow sensor fitted. Diagnostic check at main Fiat dealership identified the air flow sensor as a fault, which is why it was changed. The old sensor did have quite a lot of oil in it upon removal. Any ideas on the fault as it is annoying on the motorway when this happens and you have rev it.
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