what version of php and IIS are you running?
The Fast CGI Module in IIS 7 can encounter a number of problems with PHP if not properly optimized.
see the "PHP Process Recycling Behavior" section here:
using-fastcgi-to-host-php-applications-on-iis-7/
also this page:
how-to-configure-and-optimize-the-microsoft-web-platform-for-php/
for information on optimizing IIS with Fast CGI for php applications.