aphasia

I'm having episodes, when I cannot talk, I am conversing with someone and all of the sudden I just cannot get the next word to come out of my mouth,after a few seconds of silence,(all the while I' m moving my lips trying to articulate,but no sounds comes out) I then blurt out a sound which is not always a word and sometimes a word, but not the one I intended to say. I've had an MRI and the neuro. told me I am fine, but this speech thing keeps happening and I find myself becoming more anxious, and when I'm anxious it  gets worse.I don't know what to do next. i don't know what kind of doctor I shld see,  But I'm really worried. You see , I talk on the phone for a living,,,this is really getting difficult