So, you are making up to 150 requests in a 30 second period (10 "parallel requests", with a 2 second pause in between).  And failing at 80 to 100+ seconds of operation and 400 to 500 total requests?

 1. This [other failure mode (NOT a 503)][1] trips after about 32-ish seconds and 180 (sequential) requests.
 2. It was [previously reported that making about 2 calls per second trips a 503 in about 80 calls][2] (41-ish seconds).  That same comment also said that a 503 still occurred with intervals as long as 920ms (which I'm not sure is still true). 

So, try:

 1. Increasing the interval between batches to 30 seconds (SWAG'd number).

  OR
 1. Making your requests in "batches" of 1, with an interval of 1 second in between.  If that fails, increase to a 2 second interval.


----------
Obviously these are just workarounds, but the documented history of response to this family of bugs is not promising.  (See the linked Q&A's to start.)

  [1]: https://stackapps.com/questions/8356/after-successfully-retrieving-180-pages-the-api-gracelessly-semi-silently-fai
  [2]: https://meta.stackexchange.com/questions/249893/how-many-requests-are-too-many#comment816016_249893