I already deleted the fsx,added the highfixme=1 and reinstall the video driver... not sure what else to do.
This suggestion is only valid if you have PARTS of objects missing, like the wheels, or the driver, etc. Your problem is entirely different and it's random vehicles disappearing.
We finally were able to replicate the problem thanks to the help of some users (we are still unable to replicate it on any of our system), so it will be fixed in the next update that will be out in the next days.