This cheat will solve all your problems if you're struggling with the game. To enable it go to the 'Cheats' screen and highlight the 'Unlimited Time' option and then simultaneously press and hold L2, R1, Right and circle.
Once the game boots up, go to cheats then highlight "Unlimited Time". Next, hold down L2, R1, d-pad right, and circle. You will then have all levels done with a gold medal, all of the bonus cars unlocked, and all of the cheats unlocked. The cheats are awsome. You got to try all of them except for the one that says "Poor suspension".