If you or your visitors notice that your site is slow, then fast, then not working, the culprit is likely your site's DNS. The solution for this issue can be as simple as waiting for propagation to end or correcting your DNS glue by adding/editing a DNS record.
If you recently changed your name servers or DNS in the past 48 to 72 hours, you are currently experiencing what is known as the propagation period. Once propagation has completed, your site should be stable.
For more information on propagation, please refer the following article:
If you are using private name servers, or if you have stopped using private name servers, then you need to make your NS records in your domain's DNS zone match the new name servers you are using.
For more information on how to fix DNS glue, please see the the following article: