Getting Internal Server Error "500" null

when i try to search this blog using API explorer i get this response 

500 Internal Server Error

- Hide headers -

cache-control:  private, max-age=0
content-encoding:  gzip
content-length:  66
content-type:  application/json; charset=UTF-8
date:  Mon, 18 Nov 2013 11:06:26 GMT
expires:  Mon, 18 Nov 2013 11:06:26 GMT
server:  GSE

{
 "error": {
  "code": 500,
  "message": null
 }
}

several times and get right response few time

200 OK

- Hide headers -

cache-control:  private, max-age=0, must-revalidate, no-transform
content-encoding:  gzip
content-length:  9719
content-type:  application/json; charset=UTF-8
date:  Mon, 18 Nov 2013 11:10:43 GMT
etag:  "eW_Bw6UnvIx2fuG9gn6cxINiZWM/t3sF67tdupEdPsVjCK0Z_J4pwOk"
expires:  Mon, 18 Nov 2013 11:10:43 GMT
server:  GSE

{
 "kind": "blogger#postList",
 "nextPageToken": "CgkIChiAqN-7tycQyrTFu8G7k70l",
 "items": [
  {
   "kind": "blogger#post",
   "id": "5830508277145244399",
   "blog": {
    "id": "2700556534243482186"
   },

 why its happening

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