Arducopter highest altitude

I required to fly a quadcopter at an altitude of 1km. on an average the quadcopter should fly at an altitude of 300-500m. Is arducopter is the best choice for loiter and altitude hold?sometimes when performing the auto mission and when loitering at one point the copter seems to loose altitude. this shouldn’t happen what could I do to prevent that? and is it possible to loiter at one waypoint for more than 30mins when performing and auto mission? or I should consider using any other flight controller? please suggest me some good flight controller if you think arducopter are not suited for my applications. please let me know. thanks in advance.