seeing the poor poor perforamnce, you might concider upgrading your PC
I also fell through the texture when driving on the elevated bridge at a chemical plant map.
I've had world and entity (including cars) clipping happen occasionally. Haven't fallen through the floor yet, though, only through some parts of the level above ground, interestingly enough once happened with front wheels still colliding with it while car itself didn't. Not to mention that I'm running it with Intel i5-4690K @3.5GHz and MSI's Nvidia Geforce GTX 970 (it's bloody fine and not planning to upgrade in a long time), and 8 gigs of DDR3 RAM at 666MHz. SCi bloody fix it!
Not sure why it's happening in the official levels, but I've had this happen in my C1 level ports and that was related to the pivot positions of models relative to the bounding box of the geometry. Wonder if it's related.