Well, I hate to say it, but it sounds to me like you DO know the caause, but you are perhaps a little afraid of the scale results if you increase your calories. Really, 1600-1800/day (even with two 2000+ calorie days) is awfully low for the amount of cardio you are doing (in my completely unprofessional opinion

).
Listen to your body. Feed it with nutrient-rich foods, and I suspect you'll end the hunger/depression/fatigue cycle you are experiencing. The hunger pangs are not fake - do not ignore them.
It may just be adding in 150-250 calories extra a day, which isn't much. Or, as you suggested, take a 1 week break & return to maintenance. That time off can do wonders for us emotionally & physically!