Confidence is key. I work as Group Personal Trainer in a new studio, and for the first few weeks I was the assistant Trainer. The lead guy had a microphone on, whereas I had to shout encouragement over his Mic and the blasting music. I was so focused on physically trying to shout OVER the noise, that my body forgot to stutter. After I became the lead, that confidence just seemed to roll over. Instead of being afraid of people judging my fluency I just dove right in. My advice to you would be to keep up the good work. Don't think of the fluent days as "Rare Successes" but rather your natural voice.