I have an aggressive bearded dragon.. still...?

And I was wondering how could I calm it down. I named it draco. I've had it for about two months and at first it was nice and let me hold it... I would often take it out and just let it crawl around me. Then one day I put my hand in to pick it up and it hissed at me... the aggression increased and the first time I touched it in two weeks was today. I pet it when it was in the corner and it didn't hiss, later on when I tried to pick it up it hissed again.

For a couple of weeks I been avoiding touching Draco and just been putting my hand in the tank and waving it around slowly until it finally closes its mouth. I think it's only a few months old still. Maybe 6inches long.

One theory I have is if its too cold will it get mean? I have a lamp in one end of the tank and a heating rock (buried under sand) on the other end of the tank. The tank is about 60 gallons, I don't think its too small for Draco, however I find it running across the tank back and forth. I used to think that meant he wanted out...