I've observed this pattern with my son throughout his childhood- weeks or months with bumpy speech, then weeks or months with smooth speech, sometimes 100% smooth speech. I was never able to identify triggers, and neither is he. He's 15 now and his disfluency never vanishes entirely anymore, but still fluctuates between mostly smooth and pretty bumpy. He doesn't care about his stutter and is often not even aware of it. My son is autistic so his stutter pattern may be different than most.