The Lumia 820 for At&t got a new firmware update. It's 1232.5962.1314.0001 and it brings with it a few improvements to the performance of your device...
Improved automatic display brightness adjustment
Fix for intermittent screen blanking during a call
Improvements for connectivity issues in certain network conditions
Performance and stability improvements
You can check for the update on your Lumia 820 but going to Settings > Phone Update > Check for Updates.