DocuSign has api call limits in place - you are limited to 1000 api calls per account per hour. It looks like you are hitting this limit, which is rare so I would be curious as to how and why you are making that many calls in less than 1 hour.
The hour window is not a sliding window but rather it's from the top of each hour to the start of the next. So for instance, you can not make more than 1000 api calls in a given account between 3pm and 4pm.
Please note that this is not just for signature request api calls, but ANY api call you make against DocuSign's servers. So if you make the Login api call to retrieve your baseUrl and accountId, then that counts as one call. Then if you use that baseUrl for a subsequent signature request or other api call, that counts as 2, and so on.