This could be an awesome game
I have a superfast processor and 4 gigs of Ram which I use to render massive graphics, and even on low quality the game acted like I was on a 386. I've never had this issue before. There's something in your actionscript that's making this otherwise awesome game way too laggy. It is unplayable in its current state. Good luck with it.
Also, I think you need to re-think the keys for getting in and out of vehicles. The down arrow just doesn't feel "right." I would also suggest using WASD for movement because it's a little awkward to use the arrow keys to move and the mouse to shoot.