Excel Graph API: periodic 504 gateway timeout errors

Question

Excel Graph API: periodic 504 gateway timeout errors

My code usually works fine, but about 15% of the attempts, I get the following error:

System.Net.WebException: 
  The remote server returned an error: (504) Gateway Timeout. 
  at System.Net.WebClient.UploadDataInternal(…

My C # code does the following:

Gets a graphic token and an Excel API certificate access session (this is always successful)
Sets values in 2-8 ranges (depending on what data / calculations are needed)
Gets calculated values from multiple ranges
Gets a few updated charts

The process takes only 4-12 seconds to return all the necessary results. But sometimes one of the "setpoint" operations returns a 504 Gateway Timeout error.

Here is the code that I use to set the values (ranges of updates). It is repeated several times to update multiple ranges.

string rangeValues = "{\"values\" :[" + rowValues + "]}";
using(WebClient wc = new WebClient())
{
    wc.Headers[HttpRequestHeader.ContentType] = "application/json";
    wc.Headers[HttpRequestHeader.Authorization] = "Bearer " + Access_Token;
    wc.Headers["workbook-session-id"] = WorkBook_SessionId;
    URI = "https://graph.microsoft.com/v1.0/me/drive/root:/EwaReportWorkbooks/" +
        ToolPathAndName +
        ":/workbook/worksheets('" + sheetName +
        "')/range(address='" + rangeName + "')";
    Data = rangeValues;
    wc.Headers[HttpRequestHeader.ContentType] = "application/json";
    wc.UploadString(URI, "PATCH", Data);
}