Jaw pops and locks open and kills to close?! Help PLZ!?

So here's the problem. For about a year I've been having it. Whenever I move my jaw or open my mouth, I have this weird thing happen where I hear a popping noise and feel it like, disclocate. I talked to my dentist about it and held my jaw real tight and told me to "make" it happen. I didn't get that for 2 reasons. 1) It just randomly happens. 2) He was holding it so I couldn't. He suggested it was just too much gum-chewing. I thought so too, because I went through a pack of gum in one morning, which about 2-3 sticks in at once (sugarless). I had this habit for 1 1/2 years, which I quit about half a year ago. I haven't had any gum since then (2 sticks or so of sugarless). I had the same popping problem occionaly, and my orthodontist said it was probably from clenching my teeth, which is another of my bad habits. So just recently, it pops anytime I even swallow (a lot). This morning, though, when I got to school, it popped, and I tried closing my mouth, but it wouldn't close (cont.)