Old Email Messages

Often after 7 to 10 days running our Win2k Exchange server begins to hang or some nt services will stop running(FTP)rendering the site unavailable. Restarting the server works for another week or two. However, after the restart for 2 to 5 days,end users begin receiving previously received email messages as far back as a year or so. What can i do to help prevent a reboot and more so old emails messages?