Kinda sounds like you're just falling partially asleep, and your body is jolting itself awake again. Part of the signal your brain sends off when you're falling asleep often gets confused as "I'm dieing". So your body will jolt itself awake again. The brief half-awake period between falling asleep and spazzing awake can be like that. ^^ At least its often been like that for myself and others I know.

Also it could be you're slipping into a lucid state of sleep, where you're actually asleep but aware at the same time. That usually confuses the hell out of yourself on multiple levels, so you dart awake again. If you're finding it paired up with trouble breathing, you might have a touch of sleep apnea at work. It doesn't sound like it happens that often though. If you're curious, you could always go in for a sleep study at your local clinic that takes care of such things.

This happens to quite a few people. Especially those who attempt lucid dreaming and/or are in a midway state between being awake and asleep. For me, it sometimes feels like I fall back into my body, and I'll jump up a bit.

when your body enters REM sleep your completely paralyzed but your supposed to be unconscious. If you have trouble sleeping or getting enough sleep you go into something called REM rebound, where your brain forces you to sleep. Maybe your just experiencing an odd case of that.

It's called sleep paralysis. I had it happen to my right arm once - I could reach over and pick it up and it was like holding hands with someone else. Stress is one of the factors thought to bring it on, along with having an irregular or interrupted sleeping pattern.
The internet has more information than I can recall off the top of my head, but that's sounds to me what you're dealing with.

^-- This. First thing I thought of was this with what Grey said.

I'm obviously one of the cool kids that never gets this.

When you go to sleep your body releases a natural chemical that paralizes you so you don't move around while asleep sometimes the chemical is released early or you wake up early leaving you (temporarly) paralized, the jolt is your body trying to override the chemical.

I know this is relatively common-
My mom's mentioned it having happened to her a few times, as well as a friend or two-
though as I recall, for them is usually occurs during the process of waking up-
their brain before their body, essentially-
looks like you're doin' it backwards ^_^;

Never had it myself...so I don't know, sorry.

im not sure...but you should go to the docter...it sounds like there mgiht be somthing wrong with you nervouise sistem or spine

Sleep paralysis. It's pretty common.

http://en.wikipedia.org/wiki/Sleep_paralysis

I experienced it several times when I was around 10-12.