Yesterday I updated Mission Planner and loaded up build 39 onto the APM. This is what I observed after a full erase/setup (I have sonar and mag) -Did an initial test on the sonar to see what the new values would be by having the motors running on low throttle and holding the quad over the ground about 1.5m up. Values in the log look more accurate however the values alternate up and down about 50cm over each 20 or so log records. Looks like a mountain range.I then took it out for a fly.When I flicked the switch to flight mode and plugged in the power there is about a 5 second inactivity on the status lights followed by the A and C alternating (GPS not locked yet). As expected due to the changes in the code the APM does a level when I arm the motors. After the arming procedure lights had finished flashing the C led went out and I was left with just the A led. Could not get the motors to kick in.I then disarmed and the C led started flashing. I then armed and the C led then went solid. When I then placed the quad on the ground and lifted off it was stable however the yaw was not holding at all. It was just wondering constantly left or right depending on the direction I nudged the yaw in.I landed, disarmed, armed and took off and had the same thing.I landed, disarmed, unplugged the battery, plugged in the battery and had the same issue with the arming light going out. This time I pressed the reset button. When I did that I was able to arm however when I took off there was no stabilisation at all so could not get it to fly.I unplugged the battery. When I plugged it back in I was able to arm ok and the quad took off and was stable AND the yaw was now holding fine.I then tried alt hold at about 1m off the ground. This time it just shot up and didn't look like it would stop. Even after flicking back to stabilise it seemed to keep going so had to drop the throttle to get it back down. So it still looks like I will have to keep mucking around with the sonar placement and change over the cable so it's only shielded to ground on one end.Also noticed two of the prop savers are not perfectly centred so will see if I can eliminate the vibrations from that as well.