Unstable speech

Hi there everyone,

My question has to do with the instability of our speech when we are speaking English.

To set it up: There are days my spoken English just flows...Conversely, there are days when I struggle to find words and the rythm and intonation I use are definitely not at my best level.

I`m very curious (not to say puzzled) about the motives why this instability of speech occurs. Despite the fact one of the main factors might be the exposure to the language (ok - practice makes perfect etc), so why is that this phenomenon is experienced by people who have been living in English speaking countries for quite a while and make use of English on a daily basis?

If anyone could shed some light on this question...I`d love to hear some of your thoughts about it.

Thanks bunches