Hello Sir,
My name is Duc Nguyen, and I'm a Senior Web Developer / Linux Server Administrator. I have more than 5 years experience developing PHP applications, and more than 2 years experience with Linux Server.
From the description you provided, I guess the problem is the PHP, sometimes a mis-configured component can cause such issue. Also, there can be a problem in the code issue. I'll have to take a closer look to identify the problem.
Hope to hear from soon and to work with you.
If you have any questions, don't refrain to ask me. My skype is ducntq. I'm available to work right away.
Best regards.