M.K.
I don't know if this applies to your situation, but I will share just in case. My daughter started stuttering when she was two and half. It started so suddently that I took her to the doctor who didn't even examine her but looked at me like I was nuts. She said "Little kids stutter." I told her mine didn't before yesterday.
Anyway, I called a coincidentally nice speech therapist from the phone book. She told me little kids sometimes stutter because their physiology can't keep up with their growing vocaulary and it blips the brain, but then the brain catches up and the problem resolves. My daughter only stuttered for a few weeks, but I can appreciate how frustrating it is.
If you're concerned that the problem's been going on so long, I say ask the doctor or even call a speech pathologist to see if an evaluation is in order.