when Add/Update with 50 fields, there are error code 500 response occasionally,

I tried to add doc to index, each doc has 50 fields, each fields has at most 10 words. I tried to use SCV file and XML format, when the doc amount is small, there is no problem, but with 2 million docs to add, it fails after hundreds of add operation(10 docs each add operation). If the commit operation is performed, situation get better, but there is still error code 500 response. The reason is string index out of bound, and if the error is ignored, there will be out of memory fail.