URGENT: PHP4 /IIS4/ISAPI sometimes crashes

I use PHP4.0 as ISAPI-Modul with IIS4 and MySQL on NT4-SP5. Sometimes the PHP parser doesn't find the included files and creates an error message. Reloading the page will resolve the problem mostly, but sometimes PHP get an access voiolation and crashes. I tried to use php.exe instead, for more stability. But this causes cookies not functioning. So user access to some function of the site ist not possible. Any sugestions are welcome. Thank you Axel