i have a 1900 jtdi and sometimes when im drivin it just losses power t doesnt stall but even in 4th gear the revs drop 2 1500 and i can retrieve it sometimes by pumpin the accelorater pedal repeatedly it fires black smoke from the exhaust then goes like a rocket and can b runnin fine for days then when im in the utmost of a hurry it packs up again fiat dealer wasnt any help they said no fault was showing on it it was probably bad diesel and i should take it back in when its not workin problem is i only use my car at nights some1 please help