Constantly.
The only time I approximate "fully functional" verbally is when I'm with the few people I've known and learned to trust over many years, and even with them I generally have bad meltdowns if the conversation veers into an "emotional" subject. They've seen it before, though, and I know they won't hold it against me.
In professional circumstances, I'm usually fine so long as conversation is restricted to work/technical issues - I can discuss these intellectual challenges openly so long as things are reciprocal. If someone gets aggressive with me, however, I usually shut down and find myself only able to respond sharply, if at all.
I think one of the things that confused me for many years was the presumption by many people that becoming distraught during conversation meant that I was "processing" repressed feelings and that by "letting it out" I was somehow overcoming an imagined barrier... But that was simply never the case, because it wasn't the issue that was causing distress - it was the rawness of the interaction, and the confrontational/aggressive stance that the other person would often take.
These days I don't let people get that far inside my defenses - there's no point, since they wouldn't understand what's going on inside anyway, and I just don't need the angst/stress that comes from those kinds of conversations.
And while I wish my general conversational dyslexia (which often translates into simply not speaking at all in unfamiliar circumstance) was not so pervasive, there's really not a lot I can do to change how I feel or who I am - my mind is going to go blank whether I want it to or not.
Anyway, my two-cents.
Nick