I have recently been having the same problem with my 1998 (S) Mondeo LX TD. According to the AA and my local Ford garage, the problem is an electrical one and will cost me several hundred pounds to get sorted out by an auto electrician. Another mechanic told me, over the phone, that it sounded like an earthing problem with the battery. I am fairly sure that he is right but I have checked the terminals and they seem fine. I think that I will take it to him to look at.
I traced my most recent difficulties to the rear boot courtesy light. The car would only start when it was set into certain positions! (My digital clock also only works when the headlights are on... Interestingly, the clock also comes on when the car is immobilised.) I'm sorry that I can't be of more help but I would suggest playing about with the electrics next time it won't start.
Alex Smith, June 2009
