File reception problem with a web server service on IIS 7

I created a SOAP Web Service Server on IIS 7.5, because a client of another company has to send me files regularly by calling a specific method, according to the specifications of a .wsdl file provided by the company itself. I made the code in php, and everything seemed to work fine, I regularly receive the files ..... until I realized that my service does not receive files over a certain size. I have activated the "Failed Request Tracing Rules", and I have detected the following error steps:

1) FASTCGI_WAITING_FOR_RESPONSE

2) FASTCGI_APPLICATION_MANAGER_SHUTDOWN Error

3) SET_RESPONSE_ERROR_DESCRIPTION Warning ErrorDescription="C:\php\php-cgi.exe - The worker process must shutdown now or configuration has changed"

4) MODULE_SET_RESPONSE_ERROR_STATUS Warning ModuleName="FastCgiModule", Notification="EXECUTE_REQUEST_HANDLER", HttpStatus="500", HttpReason="Internal Server Error", HttpSubStatus="0", ErrorCode="The server machine is shutting down. (0x800704e7)", ConfigExceptionInfo=""

The problem is found on files of larger size, but we talk about 50 Kb or so: in these cases, between phase 1 and sentence 2 can pass several minutes, sometimes almost an hour!

I emphasize that in the FastCGI settings, I "fired" all timeouts to the maximum!

Does anyone have any idea what the problem might be?