Odd PHP/ErrorDocument behavior

I'm getting a little wierded out here because I'm seeing stuff on google that says others have had similar problems, but not my exact problem.
I'm in the middle of moving from an Apache1 server to an Apache2 server. Our server heavily relies on php so much that we decided to simply make even html documents (using AddType) parsed as PHP. Our ErrorDocuments use it to some extent as well (IE, the 404 doc will confirm the URI the person was aiming for, not just adding headers and footers).
The problem is that when I browse to a page that's not there, Netscape on my Mac says it needs to download the file (giving the mime type as application/x-httpd-php). On a W2KPro box, IE starts to download something but then barfs.
On the Mac, if the page is saved out, it's parsed as it should be. However, the Apache2 error log reports:

script not found or unable to stat

Anybody have any clues? Need any more information?

Eric Stewart - Network Admin - USF Tampa Library - eric@lib.usf.edu
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