I have the 520 and it's been acting up. I try and and start it, then the start up screen comes on and stays on. It won't turn off at times and other times it shuts itself off(all while stuck on the start up screen). When it doesn't turn off, I have to turn the battery switch off. After doing this as many as a 1/2 dozen times it usually starts working. It doesn't act up every time, but enough to be annoying. What can I do to fix this?