code sometimes not executing on localhost

Hi,
I have a strange problem. I've installed Apache/1.3.31 PHP 4.3.7 and MySQL on my localhost Win XP machine.

PHP is executing in most instances but sometimes it doesn't. When it doesn't I instead get the code showing in my browser window like a text file.

These files don't have short tags and I have specified [short_open_tag] to: ON

Is there something else I have missed in the php.ini?

Other then with certain files PHP seems to be working fine. I can run very detailed PHP CMS programs and E-commerce stores.

I would appreciate your help.

Thank you!

Is it certain scripts that always don't parse, or is it sporadic that a script will not work, referesh, and it does work?