Hey Tup:
yes, way back when... dissociation was pretty tightly linked to MPD; these days most professionals realize they're different animals - even though there is still some overlap of symptoms. When I was dealing with old memories that started to become conscious again, I also had to deal with a level of dissociation that returned with the memories. In some ways, the dissociation was a lot scarier than what I was remembering.
I feel like I'm repeating myself... but you need to hear this. There is a whole scale or range of dissociation. "Tuning out", being "preoccupied", or daydreaming is a technically a type of "dissociation" that most people experience rather frequently. A more intense version occurs for some people when facing a life threatening situation, or sexual assault or other trauma. In children, the fear can be very great - so much so that the usual fight/flight response can expand it's repertoire (so to speak) to include a "freeze" response - and the brain will then intervene to remove the child's senses and awareness from the situation by "going somewhere else", watching from somewhere outside of the body, going away with "angels".... and countless other child fantasies, I'm sure. This type of dissociation is involved in PTSD responses, too.
GS's suggestion of grounding exercises is the right thing, when you feel like something is coming up and about to spiral out of control. Consciously slow down your breathing - methodically tense then relax muscles from one end of your body to another - count to 10, then count backwards... hold your hands together, fingers & thumbs touching, as if cupping a small light fluffy kitten in them and rest your hands on your stomach as you breathe in - and out - of the space between your hands.... and the moment of terror passes. You can learn to re-ground yourself in the space of less time than it takes for a bathroom break. You can add things that work for you too; but the idea is that you are giving yourself a moment to be consciously aware of the fact that you ARE present - in your body - and that you ARE SAFE. in that very moment.
With old memories resurfacing, they tend to bring along a second problem - this fear of dissociation, too. For me, this was a much bigger fear than what I remembered happened to me. I was afraid I would re-experience the intense version of it and at the same I was mesmerized by it... I wanted to know what it was; study it... see if I could induce it... (ok, that's probably pretty morbid & weird!!!) and the end result was that the fear of the dissociation quickly dissapated along with my inability (and frustration about that failure) to reproduce the state. No, I couldn't make myself go into "that other world"... No, I couldn't just slide into it "accidentally" - to the point that I did in the original experience. Yes, I still remembered enough of the feeling... to be able to use it constructively, to protect myself even, when in highly charged situations. Now, I even use it to reground myself sometimes.
The flip side of my failure to reproduce the state, was total relief that I wasn't "crazy". Some experiences, feelings, memories are so traumatic that our brains go into "last resort" rescue operations for our identity/self... and they "dissociate" - or isolate, separate, wall off, cut out - the feelings, the memory, the bits of experience that go into the thoughts like "this can't be happening - to ME". That's the kind of dissociation that I think is involved with PTSD... and that kind of PTSD (again - to a degree) shows up particularly in child-victims of various kinds of trauma who have odd "gaps" in their memory (not exclusively).
For me, I was able to - this many years later - to describe in sensory detail the events of a specific day - right up until the "blank" space of that extreme dissociation. Took me a few years of therapy & journal writing & sitting with those memories, but I was able to apply what I call "emotional forensics" to all the facts I could assemble, the timelines I could firmly establish, and piece together what happened in the blank space time - not what I experienced - but what happened. And THEN, I could finally deal with all the emotions, realities, the flotsam-jetsom of that experience that was floating around loose in my psyche... and resolve, let go, and move on.
And yes, I STILL use grounding techniques!!

I think this is one of those things I should've been taught as a little kid, but wasn't.