When coming up on acid, I find that every once in a while an intense shiver will run up through my spine. It isn't prolonged, but I do sometimes have goosebumps after.

What is interesting to note is that I was tripping with a friend, and we were sitting on the same couch. At one point he went "man the acid's starting to kick in, I just had a shiver" JUST after I had shivered myself. I would assume that it is just hypersensitivity to the surroundings and that a draft came by, but I can't be positive.