oh i had similar problem the solution is the battery is loose so it keeps moving about and looses the battery connection touch on pins best thing to do is at the bottom of our N900 where the battery fits inside there are two metal strips slightly bent located preety much at the end of left and right side just bend them a bit more outwards first put your battery in then with small flat head screw driver lightly bend them towards the battery so it helps hold battery in place and if battery doesnt move about your phone wont sitch off!
hope this helps you enjoy back your n900 as this problem gets soo annoying!