Code dying at Include borders

I've got a web app that uses include() extensively - like a substitute for function calls. I learned this style of modular, file-based programming from Lasso, then converted to PHP. I like the fact that includes automatically both inherit and bequeath variables; Anyway, this web app I wrote ( a registration system for sporting events ) is having problems. Sometimes the code execution just dies, and it always dies around the borders of the include() statements. It is an intermittent problem - I've got 10 cases of code execution dying out of about 800 accesses. It only happens when feeding the pages via SSL, and it seems to only happen when the server is handling several registrations at the same time. Is there a problem with Includes where the included file is subject to some type of 1-at-a-time access limitations? The most common 'death' I've got is a little include that sends the HTML to draw some tab-like stuff. It gets called 7 times for each access, so if there are several people using the system at the same time, then the file is likely to get called almost simultaneously from more than one user. This is driving me nuts. Any suggestions? -Derrick