You'll need to do some observation and testing when the condition occurs.
Does the engine crank slower than normal when it doesn't start? A
cranking voltage reading would be useful if you can take one.
Do you have appropriate fuel pressure when the problem occurs?
Do you have spark when the problem occurs?
Do you have injector pulse?
Until you know the answers to some of these questions, you haven't
established which system(s) are and are not likely to be the problem.