Before you upgrade to NTA 3.6, consider upgrading to NPM 9.5.1. There were several fixes in 9.5.1 to the polling engine used for CBQoS and by upgrading to 9.5.1 first, you are more likely to have a smooth upgrade to NTA 3.6.
If you've already upgraded to NTA 3.6 and are experiencing issues with CBQoS polling, please review this KB article