500 Errors submitting membership TSV file

I am trying to submit a large membership tsv file (63K lines), and I've written a FISSFC command (available in v0.13.1) to batch the update into multiple smaller groups (default is 500, but configurable), as submitting large TSV files all at once is a known issue. However, I am still getting 500 errors from the importEntities endpoint:

timdef@wmc82-d41:~/tmp$ fissfc -v
0.13.1

timdef@wmc82-d41:~/tmp$ fissfc -V entity_import -C 2000 -w dev -p broad-firecloud-gdac -f patched_TCGA.2016_11_03__01_00_02.Sample_Set.loadfile.txt
Batching 63807 updates to Firecloud...
Updating sample_set memberships 1-2000, batch 1/32
Traceback (most recent call last):
  File "/usr/local/bin/fissfc", line 9, in <module>
    load_entry_point('firecloud==0.13.1', 'console_scripts', 'fissfc')()
  File "/usr/local/lib/python2.7/site-packages/firecloud/fiss.py", line 1686, in main
    sys.exit(args.func(args))
  File "/usr/local/lib/python2.7/site-packages/firecloud/fiss.py", line 144, in entity_import
    chunk_size, api_url, verbose):
  File "/usr/local/lib/python2.7/site-packages/firecloud/fiss.py", line 1143, in _batch_load
    fapi._check_response_code(r, 200)
  File "/usr/local/lib/python2.7/site-packages/firecloud/api.py", line 55, in _check_response_code
    raise FireCloudServerError(response.status_code, response.content)
firecloud.errors.FireCloudServerError: 500: {
  "statusCode": 500,
  "source": "FireCloud",
  "timestamp": 1481308448927,
  "causes": [],
  "stackTrace": [{
    "className": "spray.can.client.HttpHostConnectionSlot$$anonfun$connected$1",
    "methodName": "applyOrElse",
    "fileName": "HttpHostConnectionSlot.scala",
    "lineNumber": 148
  }, {
    "className": "akka.actor.Actor$class",
    "methodName": "aroundReceive",
    "fileName": "Actor.scala",
    "lineNumber": 480
  }, {
    "className": "spray.can.client.HttpHostConnectionSlot",
    "methodName": "aroundReceive",
    "fileName": "HttpHostConnectionSlot.scala",
    "lineNumber": 33
  }, {
    "className": "akka.actor.ActorCell",
    "methodName": "receiveMessage",
    "fileName": "ActorCell.scala",
    "lineNumber": 526
  }, {
    "className": "akka.actor.ActorCell",
    "methodName": "invoke",
    "fileName": "ActorCell.scala",
    "lineNumber": 495
  }, {
    "className": "akka.dispatch.Mailbox",
    "methodName": "processMailbox",
    "fileName": "Mailbox.scala",
    "lineNumber": 257
  }, {
    "className": "akka.dispatch.Mailbox",
    "methodName": "run",
    "fileName": "Mailbox.scala",
    "lineNumber": 224
  }, {
    "className": "akka.dispatch.Mailbox",
    "methodName": "exec",
    "fileName": "Mailbox.scala",
    "lineNumber": 234
  }, {
    "className": "scala.concurrent.forkjoin.ForkJoinTask",
    "methodName": "doExec",
    "fileName": "ForkJoinTask.java",
    "lineNumber": 260
  }, {
    "className": "scala.concurrent.forkjoin.ForkJoinPool$WorkQueue",
    "methodName": "runTask",
    "fileName": "ForkJoinPool.java",
    "lineNumber": 1339
  }, {
    "className": "scala.concurrent.forkjoin.ForkJoinPool",
    "methodName": "runWorker",
    "fileName": "ForkJoinPool.java",
    "lineNumber": 1979
  }, {
    "className": "scala.concurrent.forkjoin.ForkJoinWorkerThread",
    "methodName": "run",
    "fileName": "ForkJoinWorkerThread.java",
    "lineNumber": 107
  }],
  "message": "Service API call failed"
}


timdef@wmc82-d41:~/tmp$ fissfc -V entity_import -C 250 -w dev -p broad-firecloud-gdac -f patched_TCGA.2016_11_03__01_00_02.Sample_Set.loadfile.txt
Batching 63807 updates to Firecloud...
Updating sample_set memberships 1-250, batch 1/256
Updating sample_set memberships 251-500, batch 2/256
Traceback (most recent call last):
  File "/usr/local/bin/fissfc", line 9, in <module>
    load_entry_point('firecloud==0.13.1', 'console_scripts', 'fissfc')()
  File "/usr/local/lib/python2.7/site-packages/firecloud/fiss.py", line 1686, in main
    sys.exit(args.func(args))
  File "/usr/local/lib/python2.7/site-packages/firecloud/fiss.py", line 144, in entity_import
    chunk_size, api_url, verbose):
  File "/usr/local/lib/python2.7/site-packages/firecloud/fiss.py", line 1143, in _batch_load
    fapi._check_response_code(r, 200)
  File "/usr/local/lib/python2.7/site-packages/firecloud/api.py", line 55, in _check_response_code
    raise FireCloudServerError(response.status_code, response.content)
firecloud.errors.FireCloudServerError: 500: The server was not able to produce a timely response to your request.

How can I reliably upload an update of this size, and how can it run faster? It's still a dozen seconds or so round trip for each api call, even if it succeeds.