it's an anxiety thing, no doubt. kind of like if you and some friends got in trouble with the cops and you were suddenly in the office with a cop two inches from your face getting all the story out of you...you would probably freeze up! or when you take a test, and stuff that you could discuss so eloquently the night before over several beers, is gone from your brain when you need it.
basically your anxiety level is so high that it's hard not to feel frozen etc..so this isn't like an "oh my god this is all part of the whole dp thing"...it is simply a symptom of anxiety you are going through. just on a grand scale. but because you're anxious you tend to forget about all the times you were mentally healthy and at a loss for words, for one reason or another.
i think that's what a lot of these symptoms are...we have them all the time anyway, even when healthy. looking back i've realized i've always had a sleeping problem, and i've always been tired, and i've had times of bad memory or not knowing the words or feeling a little detached or out of it or numb...all when I didn't have DP. it's just that when it gets so bad we think of it exclusively as part of teh dp instead of realizing that it's a totally normal human reaction related to anxiety.
edit: the reason why i'm trying to show that these are simply anxiety symptoms is that for a long time i have made myself worse by thinking of things in terms of black and white: "this IS the dp, that is NOT the dp", kind of like everything had to get better all at once for me to be a complete human again. kind of like i was on one side of reality and had to find the exact code to get back to the other. but seeing these things as anxiety symptoms gone awry helps me realize that "hey, i'm still here, i just have so much mental fatigue and anxiety going on that my mind is playing a few tricks."