Same for me, and recently I've been considering a connection to overall CNS fatigue. I've been working out a lot harder than usual in the last year or so, and when I go 6+ days straight without a rest day, my stuttering has usually gotten much worse. Then a day or two of rest and my speech bounces back. Hasn't been long enough to 100% connect it but since sleep has always been an indicator for me, I wouldn't be surprised if this is too, as it's a similar idea.