C#下载报错：An exception occurred during a WebClient request.，不知道如何处理。

//如果下载失败会请求3次
string exep = "";
do
{
try
{
if (!Directory.Exists(Dir))
{
Directory.CreateDirectory(Dir);
}
using (var wc = new WebClient())
{
wc.Credentials = CredentialCache.DefaultCredentials;
wc.Headers.Add(HttpRequestHeader.UserAgent, "anything");
wc.DownloadFile(strFullName, Dir + strFullName.Substring(strFullName.LastIndexOf('/') + 1));
}
return true;
}
catch (System.Net.WebException ex)
{
//图片服务器上也存在也会报错
if (ex.Message == "An exception occurred during a WebClient request.")
{
exep = "occurred";
}
else {
exep = "";
}
}
catch (Exception ex)
{
LogMsg.WriteLogToApplicationFolderByMonth(ex.ToString() + "-------------------------------------FileURL：" + strFullName);
return false;
}
} while (exep == "occurred");
return true;

图片在服务器存在，能够正常访问，但是下载的时候时不时会报“An exception occurred during a WebClient request.”

不知道如何处理，所以就做了一个循环，最多下砸3次。（目前可以解决下载的问题，因为这个问题是偶尔发生在某一个下载请求里，所以用了个循环请求直至3次请求下载完成）