I am not sure what is causing this, so let me explain.

CentOS 5.3 i686
cPanel 11.24.5-R37629
WHM 11.24.2 - X 3.9
Apache 2.2.11
Php version 5.2.9

Now the problem lies with two files getting "corrupted" consistently. I am running a Phpbb forum version 2.0.22. The two files that are getting corrupted are the index.php file and the /admin/index.php file. It's almost as if the last section of these two files are just being deleted. If I upload backup copies of these files, the site works just fine. Usually over a period of a few days, the same thing occurs.

I've looked through the access log, but it's so large I can't seem to find anything. What should I be looking for exactly? Anyone have any idea what might be causing this?

I believe the permissions on those two files are set at: 644

Unfortunately you chose to run a vulnerable version. Any "workarounds" or "temporary fixes" will only treat symptoms. Please address the cause before doing anything else.

I just realized that I was running an older version when I went to check which version it was, it told me it was out dated. Though I have to ask, is the vulnerabilities in phpbb 2.0.22 causing these issues? Is it foul play? Something wrong with the setup? Just patching/updating an issue isn't really solving anything either, I'd much rather know exactly what is causing it.

Current stable is 3.0.5 (2009/May/31). If you "just checked" then you'd also like to know that security maintenance of phpbb2 code ceased on 2009/Feb/1. If you understand the problem of running stale, deprecated and vulnerable software then you know you have to address that before doing anything else and instead of trying to come up with counterarguments.