Intermittent issues are the hardest because conflict testing may not catch them. The cause is usually dynamic: caching serving stale data, optimization plugins interfering with AJAX, server performance under load, WP-Cron collisions, SSL renewal gaps, CDN differences between cached/uncached requests, or browser caching on the customer’s end.
Diagnostic toolkit: Disable caching/optimization plugins first. Check WooCommerce → Status → Logs for errors with timestamps — correlate with reported issue times. Use browser dev tools (Console + Network tabs). Install Query Monitor. Check Settings → Permalinks → Save for routing issues. And document the pattern — time of day, specific product, specific browser. Intermittent issues are solved by pattern recognition.