What ESC, motor, battery are you running?
I would really try to find any binding in the drivetrain. Take your whole car apart and look at the bearings and clean the diffs. A thing that every person tells each other and me at my local track is this "In order to be fast you have to be slow". meaning that you cant be fast unless you drive clean and smooth. Dont go around every turn smashing the throttle and go wide around the turns. Get a good setup and just drive. Go to the Losi site for a good setup. Or you can e-mail one of the Losi guys and they would gladly be able to help. They answere your e-mails fast especially Hodge.
Hope you get your car running great. The XXX-S is a great car and have never had a problem with it.