Yeah, I have the same thing happen to me... out of the blue, while I'm just sitting on a chair chilling out, no stressors happening, I go, like you say from 60 to 0 in a minute. I call it a 'slump' because suddenly I feel I'm too fatigued to sit on the chair, like I could slide off of it to the floor suddenly.Also I get spacey and brain fogged and unmotivated...like blah. This is quite different than a low blood Sugar episode, though, the symptoms are completely different.
The funny thing is that if this is going to happen, it usually happens like clockwork at 10:30 each day, and lasts until about noon. However, sometimes it happens out of the blue at any given time, with nothing odd precipitating it.