So, your site is slow, then fast. It works, then it doesn't work. If this is happening to you, the likely culprit is DNS.
If you recently changed your name servers or DNS, then you are in a 48 to 72 hour period known as propagation. Once propagation is over, your site should be stable.
If you are using private name servers, or 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.
If neither of those situations apply to you, please contact support so we can evaluate the situation.