Hints:
1. Beware of new and improved.
2. That which is most difficult to replace, is usually the problem.
3. When all else has been replaced, that which remains, no matter how
improbable, is the culprit.

I agree with that but you're using a misleading singular.

THAT.......is THE problem.......... falsely implies only one thing is wrong. I've been fooled a number of times because two things were bad simultaneously. Sometimes one caused the other but sometimes it's just random.

In fact there is no theoretical limit to how many things can go wrong at once.