Hmm, well I always max my cars out with stars before I start work on them with parts. That being said it's obvious I had to win both level 16 and 17 for the Ferrari and McLaren Blueprints every day for over a month, plus I was buying them from chopshop.
Anyways as to your question I could no way beat 'Fast Track' on level 18 with the cars I had. I have nearly finshed both the Ferrari and McLaren totally but they don't massively have that much more PR than the car I was using which poses the question...Will I be able to beat it with my new cars or have the same issue as you?
I apologise for this not being a total solution for you!
*edit* Nope still cannot beat level 18 with McLaren! Only way would be to get lucky and take out the car who is giving you the most problems then hopefully romp home before being taken over again!
*edit*
Beat it although it was with some luck. Still at least I don't HAVE to win the race again....I mean I can either use 'Autodrive' or 'Race Skip'.