Big Bob

hi there i am after some help on trying to sort out my 2.0 sport! the prob that i am havin is that every now and then it just wont start! it doesnt seem to be firing! i have changed the coil and upgraded the plugs and leads she was fine 4 a couple of weeks then bang she wont start again! the strange thing is she will then only start if you tow it! it is realy a pain up the *** has any 1 got a clue what the prob mite be so i dont have to take it to scoob!
thanx