REST API throws "408 Request Time-out" error

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My client is using RESP API to create record in OSvC from EBS. They have loop which call POST operation to create each Orgs record in OSvC. According to them everything goes well for few record and then they start getting "408 Request Time-out" error. 

When they rerun their program it works for failed record. 

Here is the header, they have added Keep-alive

req := utl_http.begin_request(v_url, v_method, 'HTTP/1.1'); -- 'POST',
utl_http.set_header(req, 'user-agent', 'mozilla/4.0');
utl_http.set_header(req, 'Accept', 'application/json'); -- 'application/json'
utl_http.set_header(req, 'Content-Language', 'en-US');