ya, you can still play thru the cop chases, but the checkpoint based challenges are nearly impossible at some points. what you can do if you're playing a challenge, make a mental note of where the game freezes, or where you think it will freeze based on the missing buildings/stuff. push pause (+) right before you get there. if you do it right, it will lock up just as it goes to the pause menu, so it can load the rest of that part of the level and then goes directly to pause so that you can then resume without your car stalling out and flashing like it was being beamed into outer space. it's also weird how the camera suddenly turns around when that happens...
seems like these new games that render downtown areas in high detail at high speed are prone to slowness and glitches, either that or the makers of gta iv and nfs need to better optimize their games. it could also be that some hardware is just not fast enough to support all of those models in memory at once, or to load all the news ones fast enough. it would explain why it mainly has some freeze issues when a lot of buildings are being rendered.