I don't really know why, but I experience something similar. When speaking in German (mother tongue) I most likely stutter alot. I also get blocks and try to start a sentence with synonyms or filler words, resulting in me getting a stutter. However, it is much easier for me to speak in English. I tend to speak slowly and with me being more concentrated on what to say, it is somehow easier to speak fluently. My theory is that speaking a different language requires more brain activity and this is the reason why you're more concentrated. A second language has a different rythm as well which might be another contributing factor.