Are you exercising? I suggest increasing the exercise AND the calories.
Not by a lot...but I feel like you started out too low. I have no problems with low calorie dieting, except when it's not working.
(And no, 20 pounds is nothing to sneeze at! It is working!) But I hear you saying you gain at 1200 and that isn't normal.
I've just gone round about with a similar experience myself. I'm in the middle of a two month long plateau during which I decreased the calories to a point so low I had to take a multi-vitamin to stay awake.
I would lose a good three pounds, and then as soon as I ate over my calorie count the pounds came right back.
Just last week I bit the bullet and decided to increase the intensity and length of my exercise routine AND increase the calories. It was a great decision. My energy has returned, I'm no longer starving and I don't think about food. I hate thinking about food...it generally means I'm not getting enough of it! The scale is moving again, slowly, but steadily. I've lost a pound and a half in a week and a half, and I feel like it's a good loss, not a fluctuation loss. I've steadily lost a few ounces every day.
More importantly than that...stick with it.
Stick with it, stick with it, stick with it! I feel like you've made weight loss too hard and you're feeling like you're broken, looking to your thyroid or PCOS. I've been there! I always made it too hard, thought I was broken and then quit! And then I decided I really wanted to know where I'd be after a year's time if I just didn't quit. That's what's gotten me through the hard times. I'm not broken and you aren't either. You just haven't found what works for you yet, that's all.