What option are you looking for? I'm not sure I follow.
Have you captured the logs on the box when the problem is happening? It sounds kinda like a memory leak in one of the apps. Are you running them in thier own space or in shared space?
Watching the perf logs during a crash would probably start you in the right direction.
Is IIS taking a long time to restart when you finally restart it? It should only take a couple of seconds if you do it from the command line. Unless the sites on that server need to maintain state at all times, I'd probably just set up a cron job to restart IIS every night at 3:00AM until I figured out what the problem is. I might even leave it there after I fixed this problem just to clear everything out every night. Can't hurt.