Your question is Documenting a Hard-to-Reproduce Bug. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Tell me about a time you found a bug that was difficult to reproduce consistently. How did you document it, how did you work with engineering to narrow it down, and what was the outcome?