cappells

Hello, Before posting this I have checked other post of the forum where talk about this error message where is fixed by modifying the php.ini file or by just modifying the memory_limit by hosting provider. The truth is that I have a VPS and even changing the value to 512M the error only appears when customer tries to buy more than 3 products (with 3 products or less everything works fine). But when the customer tries to order more than 3 products the "Fatal error: Allowed memory size" appears changing the name of the file and I can't even access to that missing order through the BO. I have also tried moving all the modules to a new folder and adding them one by one but I still face the same issue. What could be happening? I have version 1.6.1.6