I'm also able to learn songs by ear just fine and have some trouble with intervals in ear training..
I guess when I'm learning songs by ear it's more of a trial and error type of thing...
I would really like it to be more direct...
I've used the reference songs trick and it helps somewhat. But what REALLY helps is learning to sing the intervals. I just tarted to do this and I'm noticing improvement straight away.
Get a good tuner, there are some great tuner apps for android that work very good for this, play an interval, and sing it to the tuner until you get the notes right
