Is this TMJ or is my Jaw Disc slipping? MRI?

I was tight in my neck and shoulders one evening and laid down with a warm bean bag under my neck. I fell asleep and the next morning I could not open my mouth fully. It felt as though it was hitting something solid on the right side of my jaw. I wiggled my jaw with my hand and it seamed to 'go back in.' It was fine for about a minute and then it clicked and and I could not open my mouth again. It really feels as though it is up against something solid. I went to a doctor who said I must have yawned to wide (I was not yawning!) and told me about TMJ. He gave me some mussle relaxants for a week. They made no difference.
I have tried ice/heat as well as anti-inflamatories. Three weeks later I woke up and I could open my mouth fully, but with clicking and the feeling that it wanted to 'fall out' again. I tried to stay relaxed but about an hour later whilst driving to work, without any warning or event such as chewing or clicking, it suddenly happened again. It is not painful unless i try to open it wide enough to eat. The next morning I could open it again. I was very careful not to keep "trying" it out, but a few hours into my day I noticed, once again, that I could not open my mouth.
It stayed like this for another week.
Today I got up, and it was absolutely fine. (A minor clicking but that was all)
Six hours into my day, whilst I was painting a door, I looked up to speak and all of a sudden I could not open my mouth again! once again there was no 'event'

I have had my jaw X-Rayed and been told that there is nothing wrong with the jaw mechanism itself. It does feel very mechanical and not the Charlie horse everybody says is TMJ.
Should I get an MRI to see if the disc is slipping out of place or is this TMJ and I should keep trying to relax it away?

Please help.