Yesterday, i added a new senor onto my quad, MaxSonar MB1260 XL for better altitude hold. Before attaching and re-uploading the ArduCopter 2.7.3 onto the APM 2. I erased and reset the board. And then attached sonar as described in the wiki manual. And uploaded the code and followed all initial steps (Of course i enabled the sonar). Everything went smooth.
Next step, time for test fly. It flew great in Stable mode. However, as soon as i toggled the ALT_Hold switch ON. Quad started acting strange. It kept climbing higher n higher and suddenly dropped out of nowhere. Luckily i managed to control by toggling the switch back to stable mode.
I have checked everything couple of times now. All connection looks good. Mission Planner looks ok. I don't know what the problem. Your help would be so much helpful. I have attached couple of videos below. Please take a look at them. If you need any other information let me know. Thank you.