Failure under load on NT Server

Hi, I am running PHP 4 (release) on an NT Server sp6 with IIS and SQL Server 7.0. I am using ODBC in order to connect to SQL Server. The system works well under light load, however under heavier loads (25 simultaneous connections) I get an error in the browser ('PHP has experienced an access violation 87xxxx'). I am running PHP as an ISAPI extension. Can anyone help as this is running on a live server and we are expecting a heavy load shortly?? PHP is running on the SQL Server machine along with IIS - do you think that it would help to move the webserver and PHP to another machine? Thanks in anticipation - I have been lurking on the list for a while and I know if anyone can help it is you guys. Cheers, Matt