Interesting observation -- I never would have described myself or my fellow INFJ female friends as "flaky" (normally that's a designation I reserve for my dear ENTPs!) :-0
That said....I've gotten busted a number or times with others for "checking-out" when I do not feel motivated or inspired to do something or be somewhere. If I feel a strong intuition to do/not do something that I feel will best promote my emotional well-being, or my need to be true to myself or others, practicality and duty will have a diminished voice in overriding that decision.
And maybe the flip side is that, once I've decided on a certain emotional course that I intuitively decided will fulfill some inspiration I felt, the J in me compels me to see it through. Hence sticking in non-productive relationships that I have convinced myself I want, even if rationally I know in the back of my mind they are not the right thing....
Also, I *hate* feeling controlled or bossed around by arbitrary authority. And I really hate watching bossypants people do it to others. Normally I am so diplomatic and graceful in dealing with people, and everyone thinks I have this zen sense. But you will see a very unexpected, if not aggressive, side of me come forward when I am defending against perceived bullying towards myself or others. I will verbally rip your head off if I feel inappropriately controlled.
In that sense -- the desire for autonomy and space to be intellectually and creatively free -- I think we are kindred spirits to you ENTPs! We make good sidekicks in staging rebellions against SJs....
If they're females, they can be very flaky unless they fall head-over-heels for no objectively obvious reason. Then they come up with some reason in their head to stick with a relationship that is really unhealthy or should never have happened in the first place. And after everything goes wrong, they get really jaded.
Both: Both types are notoriously pigheaded and hate to feel "controlled".