Problem with file_exists()

Hi, I've been reported a weird problem with one of the scripts that we have, which uses the file_exists() function. It returned true once, and then the query ran again, and it reported false, on the same file. This was true for a lot of files in the same directory, but only over a cerain period of time. As I had time to try to investigate, the problem stopped occuring. This script has been used for a long time, and this is the first time that this has happened. I've checked some of the files that they were having problems with, and they have not been modified for a few days, so I doubt they were locked or anything like that. Does anyone have any idea what could be causing this? This is running Solaris 8, Apache 1.3.26 with PHP 4.2.1 compiled into it. I've also checked file permissions and they were all good. Any idea would be appreciated! Thanks a lot, Steve Johnson