1) The reason the car pops up everywhere is becuase you probably edited the default.ide while installing it, which tells what group car is in. ie a rich persons car, or workers car.
2) When going fast your video card has to load up the textures faster than when your just normally driving. So if your driving real fast with an original car it can load them quick enough, but if you had a modded car that consists of a lot of pollies your video card cant load the textures fast enough.
Also if it is slow with a regular car make sure your drivers are up to date and your pc is mid range between the minimal specs and recommended specs.