weird random parse errors on PHP 4.1.x / Win2k

Has anyone seen anything like this before? I have a client running PHP on Win2k on a whopper of a machine (dual Xeon, 1GB RAM, RAID5), and when two or more people request pages simultaneously, users will occasionally get a "parse error in file.php on line X" instead of the page they should have gotten. Clicking refresh on their browser usually seems to do the trick to get the correct page back up. There are no real parse errors in any of the files, and the file & line number is different every time the error occurs. The more people accessing the system concurrently, the more frequently it seems to happen. This happens with the PHP module for both IIS and Apache. I've tried CGI, but that seems to be worse (redirects don't work right at all, for example). Any suggestions? Geoff