Yep. I found once I got ~75% through the story that the cars just stopped spawning certain models. The only real solutions I've found/heard:
* Complete restart of the console(usually works for awhile, then fucks up again)
* Start a separate new-game; play up until you can drive around free and then re-load your old save(this one seems to work the longest, but eventually you need to re-do it/restart the console)
* For specific cars(F620 etc), do the mission that has them available, fail it, and then when you go back to your normal save it should spawn more frequently(I went from seeing no F620s to seeing 5 in the last couple hours).