The WHOIS service is limited to 10 queries per second on both port 43 and for HTTP access from one source IP.
If you query more than 10 a second you will be black listed.
A better way to check the availability of a domain name is to use the zone files. You can request Donuts TLD zone files here: https://czds.icann.org/en
Donuts does not white-list IP addresses for Whois queries. Instead Whois is rate limited at ten requests per second. Ensure that connections are closed after execution as the rate the limit is connection based.
If you continue to be affected by rate limiting, and are not hitting the 10 queries per second ceiling, please contact technical support providing the IP address you are connecting from, and a copy of your log file.