DD slept great for the first 7 weeks of her life and then she went through a massive growth spurt (ended up off the chart for height/length) and was no longer able to stay asleep for more than 20 minutes at a time. I could set my watch by her - she would literally fly up in the air at 20 minutes and start crying when she landed back on the mattress. She would occasionally slip in a 40 minute cycle but that was it. Luckily she never developed that overtired grumpiness that most kids lacking sleep get.
At 14 months we discovered that she had sleep apnea - the way the docs explained it to me as soon as she would hit REM sleep her muscles would relax and her adenoids would drop, blocking her breathing passages. The flying off her bed was her brain shocking her awake so she could breathe. After repeating this cycle over and over (sometimes a dozen or more times) she would eventually be able to sleep for a few hours but as I recall it was rarely more than about 4 at a stretch.
I found the book "The No Cry Sleep Solution" very helpful. It discusses baby and child sleep cycles and gives very practical advice for how to get your little ones to sleep better i.e. lighting, routines, signs of acid reflux, etc.
I found that I had to lay down with her and if I handled it just right I could get her immediately back to sleep after she had that startle response. I think she was really just entering the next sleep cycle but I was preventing her from coming fully awake in between.
Not sure if this is your DD's problem but may be worth considering. Good luck!
