code is executing some time but some times not

private void searchAndParse(string searchStr)
{
Dictionary<string, string> queryPara;
Dictionary<string, string> headerDict;
string curGoogleSearchUrl;
string curSearchRespHtml;
string curReferer;

string googleUrl = "http://www.google.co.in/";
string tmpRespHtml = crifanLib.getUrlRespHtml(googleUrl);

string firstGoogleSearchUrl = "http://www.google.co.in/search?";
queryPara = new Dictionary<string, string>();
queryPara.Add("newwindow", "1");
queryPara.Add("safe", "strict");
queryPara.Add("source", "hp");
queryPara.Add("q", searchStr);
queryPara.Add("btnK", "Google Search");

queryPara.Add("site", "");
firstGoogleSearchUrl += crifanLib.quoteParas(queryPara, false);

curReferer = googleUrl;
curGoogleSearchUrl = firstGoogleSearchUrl;

while(needContinueSearch)
{
headerDict = new Dictionary<string,string>();
headerDict.Add("referer", curReferer);
curSearchRespHtml = crifanLib.getUrlRespHtml(curGoogleSearchUrl, headerDict: headerDict);

parseSinglePageHtml(curSearchRespHtml);
if (!needContinueSearch)
{
break;
}

HtmlAgilityPack.HtmlDocument htmlDoc = crifanLib.htmlToHtmlDoc(curSearchRespHtml);
HtmlNode rootHtmlNode = htmlDoc.DocumentNode;
HtmlNode nextHtmlNode = rootHtmlNode.SelectSingleNode("//a[@id='pnnext' and @class='pn']");
if(nextHtmlNode != null)
{
curReferer = curGoogleSearchUrl;

string hrefStr = nextHtmlNode.Attributes["href"].Value;
string encodedUrl = "http://www.google.co.in" + hrefStr;

string htmlDecoded = HttpUtility.HtmlDecode(encodedUrl);

curGoogleSearchUrl = htmlDecoded;

needContinueSearch = true;
}
else
{
needContinueSearch = false;
break;
}

}
}