Hi Linda, it sounds a lot like sciatica to me, but there are allot of things that can cause this, I find it interesting that it is cyclic and comes and goes like that, there are conditions that can cause that too, when your pain goes away, do you find that you have any deficits or problems left after it is gone, i.e. loss of a reflex or unable to do certain movements and stuff like that? I guess I would suggest that you start keeping a journal of your pain, and what you were doing when it first started, and when it starts to let up, and pay attention to any problems you might have when the pain is gone.
I personally think that an MRI would be the best thing to request, they show the most information, and especially soft tissue injury or damage. X-rays are good for bones, and maybe even preferable, but not nearly as good for soft tissue as the MRI is. If your having a nerve impingement in your back or issues with your spine and central nervous system then the MRI is the thing to have. Also nerve conduction studies might be helpful, talk with your doctor, I would think a neurology consult would be really helpful too. Something to ponder anyway. The main thing is to talk with your doctor about your problems and concern, and if he/she is not addressing your problems adequately or to your liking then definitely seek a second opinion with somebody else.
I wish you well and hope you will keep us informed of your progress.