hi there; we are receiving the error" {"error":"invalid_grant","error_description":"authentication failure"} , this is happenong only sometimes

This error can happen for several reasons. Below are check points.

• Users may not be allowed to self-authorize
• The base URL is incorrect (test.salesforce.com for sandbox or login.salesforce.com for production)
• Check to see if your base URL is incorrect, if so, that's an easy fix. If your base URL is correct and you are still getting the authentication failure error, you can try allowing all users to self-authorize.
•  Include your API security token with the password

If above doesn't help let me know your mock request and response so that we can check further.