Users report that a cloud-hosted web application intermittently fails to load large report pages, while small pages and API calls always succeed. The application sits behind a Layer 7 load balancer. Backend server CPU, memory, and disk metrics are all normal, and health checks consistently pass. Access logs on the backend show that some requests complete while others are silently truncated mid-response. Following a structured troubleshooting methodology, which action should you take FIRST to isolate the root cause?