As you have said, it might be a problem with the type of file extension (shtml,html,php) and how the server treats each one.
But you mention that you're getting different results, depending on where the file is put. For you it works better in some locations than in others. This sort of thing tends to happen when the paths to the other files are wrong.
Some paths are 'relative' (if you move the file, it won't be able to find the other file if you don't move them to a similar location in relation to where it was before).
Analogy: If you live at No.10 to get to No.8 you just go next door. But if you move to No.20 you can't just go next door, otherwise you'll be at 18.
Some paths are 'absolute', either because you're using the exact file location such as the full domain name, path and file name OR because you're using the exact file location of your internal files.
PHP Code:
<?php
include("/var/www/html/content_cars.html");
?>
Would work no matter where you called the file from.
D