hi my dad has a 95 land rover discovery 3.9 V8.. and its surging while idling like the revs just keep going up and down to the point where it stalls.. so far taken it to a mech twice and first time had it full serviced+tune, and after 2weeks still doing it and the plugs were covered in black so took it back.. 2nd time plugs and leads changed, injectors cleaned, T/B cleaned. still the same problem been about 3 weeks and the plugs r stuffed again same prob tips r covered in black shit. any ideas wat could b wrong.. the mechanics dont seem 2 have a clue.. thanks