I've had the above problem for a while now and not been able to figure it out. I tried all the cookie suggestions but nothing.
What happened was that FireFox sometimes downloaded the php files instead of executing them and then displayed php garbage file locally on my computer.
I don´t know if anyone else has had the same problem but I thought I might post the solution.
The Apache server configurations had to be changed. These are the ones that were replaced:
php_admin_flag engine off
AddHandler x-httpd-php .php
suPHP_UserGroup levy users
//<i>Levy is my user directory at my ISP</i>
php_admin_flag safe_mode 0
p.s. I obviously take no responsability if your server crashes. :)