Web Service Object Refrence not set error

Hi ,
I have created a webservice which return a large amount of data it's working fine
but if i call it 1000 time it's given out put only for 600 after that it's given an error
<?xml version="1.0" encoding="utf-8"?><soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><soap:Body><soap:Fault><faultcode>soap:Server</faultcode><faultstring>System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> System.NullReferenceException: Object reference not set to an instance of an object.    at GetQuoteFromXml(String objQuoteRequestXML) in TestEngine.cs:line 411    --- End of inner exception stack trace ---</faultstring><detail /></soap:Fault></soap:Body></soap:Envelope> ...