Sept. 9, 2017, 6:59 p.m.
I'm not completely sure but I think it's caused by cars that have too many polygons/details. In Underground 2 it was also caused by cars that had textures that were too big.
So the only thing I do myself is if I notice that a car might cause that problem, I remove that mod (unfortunately it's the only way around it I have). If you see some specific car in many races where the problem coincidentally happens, maybe that car is causing it.
For Underground 2 I think I got around the problem when cars had very big textures, by exporting the textures, editing the big ones to make them smaller, and recompiling the texture file, but it was a bit tricky and maybe not worth it for Most Wanted, if it's not the real cause. Let me know if you want me to tell you how to do that though.
In MW I get the problem in 2 computers but they aren't gamer computers, so Idk if it's a computer specs problem.
EDIT: I was just checking some car mod for MW and in the comments someone asked about this problem too, and someone replied "try setting car geometry detail to low" (in options). Maybe that works? Gotta try it.