allocate customer payments

I have a list of customers, which i usually select to allocate off the customer transaction. There is one customer with a long list, I can't seem to allocate the funds and process it. I am using a linux server with apache and mysql. Wonder is there is a specific memory allocation for the process of allocation customer payment. Where as if I use it on xamp, it works fine.

So, wondering if there is any specific guidelines of memory allocation for linux server.

Not sure if this is the right place to put the topic either. But so far, only the allocate customer payment or credit note is giving me the problem.