Simulate a scenario where a high-traffic web application intermittently returns 503 Service Unavailable errors during peak hours. Guide me through a Root Cause Analysis using the 'Five Whys' technique. For each 'Why', propose potential diagnostic logs or metrics one should check to confirm or deny the hypothesis before moving to the next level. Assume the stack involves Nginx, Node.js, Redis, and PostgreSQL. Conclude with a hypothesis for the underlying cause and a remediation plan.