Unisphere REST API 401 and/or request limits?

Is there a limit on the number of requests that can be made to the REST API in any given period?

I am trying to get information from about 1000 storage groups, and I am getting random HTTP 401 (Unauthorized) errors when attempting to collect. The credentials are good - they're the same for every request, and it seems that it's not failing on the same request every time. I'm wondering if I'm hitting a call limit imposed by the server? Or perhaps there may be another reason for these random 401 errors?