[MENTION=14807]Millis[/MENTION],
Without being too morbid about it, I'm curious what exactly were the issues that precipitated these break ups. ENTJs take commitment fairly seriously, and are fairly all or nothing about those we make. For me to want to break it off with someone, it's probably because somewhere along the way that person has violated some boundary of mine enough times that I feel I can no longer tolerate them. I'm guessing that your ENTJ has taken exception to some pattern in your behavior, and that he hasn't adequately articulated what that pattern is or you haven't listened to him when he's told you what's wrong. The upshot in all this is that he clearly does care for you, or he wouldn't have come back to you.
Another thing is that ENTJs typically don't shy from conflict. This idea you've picked up that he either can't accept that you guys fight or can't accept the "imperfections" in your relationship is probably not exactly the case. Unless he's stated as much to you explicitly, or is actually an ENFJ. We don't really seek for people to be perfect, and while we can be demanding in certain spheres, in our relationships there are few who are more accepting. What we don't tend to accept are behaviors we know to be beneath the person we care about (i.e., self-destructiveness, wasted potential, ect).
All this being said, I'd recommend that rather than speculating with strangers, talk to your ENTJ directly and ask him what does he need from you in order to be happy in your relationship. If what he requests is reasonable, then you guys can begin working on it together. If you don't think you can give him what he needs, then move on.
One thing I do know about NJs is they always have a "Vision" of what they want for the future - sort of an invisible set of criteria - and if you stray too far from the boundaries of that Vision, they won't be interested in you anymore. My guess is what may have happened when he broke up with you "spontaneously" was that it wasn't really spontaneous at all in his head, and that it was probably the point at which he decided you weren't meeting his criteria. For a relationship with an NJ to work, you have to make sure that his Vision includes allowing you room to make mistakes and grow while still being a good partner for him. He will need to clarify what he must have from you at all times and what can be more negotiable, and he will need to make room to allow you to be who you are and grow in the directions you want to grow. It's possible that who you are is simply incompatible with his relationship needs - that's happened to myself and an NJ before, and it's painful, but it would have been more painful to stay in the relationship and always confine myself.
I initially quoted your post to disagree with your point, Sky, only to contribute something of a variation on a theme. All the same, I think this notion of "vision" doesn't apply so much to ENTJs. We're more "planners" than "visionaries" per se. Whatever grand designs Ni cranks out are essentially working documents that can change on a dime depending on the feedback absorbed via Se. That contrasts sharply with an INTJ, for instance, whose Ni is anchored in Fi instead. Those visions are an expression of their being, and going against that is tantamount to rejection in their minds. That ain't our hang up, though.
Where you can go wrong with an ENTJ as an Ne-dom is by undermining Te's need to say that something
is. Even on an abstract level, we like to be able to pin something down as being one way or another. Ne-ing something excessively can be misinterpreted as dishonesty, and taken very personally. We can learn to accept that, however, as we begin to understand the rules of the game. My assumption in my response is that the OP has committed an Fi violation rather than an Ni one simply because her SO's behavior is clearly less methodical and more emotionally-driven than if she was simply derailing his plans.