Yes, that happens to me a lot! You feel that your mind gets locked and you can't think of the right word (or maybe any words) and your grammar is terrible. You can hear mistakes even as they leave your mouth.
I've noticed that my language ability changes due to these factors:
1) how comfortable I feel with the person I'm talking to, or with the situation.
2) how knowledgeable I am (or feel I am) about the topic I am discussing
3) if I am stressed or under time pressure
4) if I have been speaking another language for longer (example: If I have been speaking Italian for a few hours and I start to speak Dutch....the Dutch might not be so good)
5) if I have to speak that language suddenly and don't have a minute or two to prepare for it.
6) if I have been speaking for a longer period of time and I am tired. Speaking a foreign language is like using batteries....if you speak for a longer time, you can feel the 'batteries' getting weaker and weaker...
Speaking ability is not only about what you know, or your skills. Emotional and psychological factors are also very important!