I am new to this forum and would like to add my similar experience to this thread.
I too have issues with the "H" responding to the ST-16 on throttle down to descend. Calibrations (compass, accelerometer, gimbal all successful). Most recent firmware; autopilot is 1.34. Updated by Yuneec at the last repair visit in March. Always in Angle mode, always rabbit mode. Have a Q500, Q500-4K, Q500-G that I have been flying for a couple years. All three are absolutely dependable and reliable. No hesitation flying over water or forest. The "H" arrived in early December 2016. I have yet to have a successful satisfactory flight. Yuneec has taken it back twice to correct problems and it still isn't fixed.
The first time it was sent back for going into uncontrollable spirals, increasing speed and radius. Yuneec replaced the GPS unit and sent it back. It arrived back in December. Didn't fly again until late February. I'm in Maine. The weather was too cold, windy, snowy to fly. First warmer, clear day in February I took it out for a test flight expecting my first satisfactory flight. I let it hover at several altitudes. Good! New GPS is working. Moved it around some, looking good! Tried to descend, No response, instead it began to move erratically in circles alternately changing altitude by several feet up down (Angle mode, rabbit). When it was over deep snow I hit the motor stop button fearing it could take off at any moment. Came down about 20 feet before the motors stopped (I am aware of the delayed stop action) and it fell about 15 feet into the snow. Broken landing gear retractor unit; loose camera. Reviewed the telemetry and noted that the RSSI value (Received Signal strength Indicator) had many zeros at the time I tried to descend. A Zero means "No signal received" prior to that the signal strength was between -30 and -35, excellent signal. Sent the telemetry to Yuneec. They agreed there was a problem, issued a Return Authorization. It went to the outsourced repair facility in Kansas. Several weeks later it was returned. I opened the telemetry file from the test flight in Kansas. Still many intermittent zeros in RSSI. Tried my own test flight. Hovered okay for a couple minutes then on throttle down it began to "go crazy" again. Hit emergency stop before it took off on me. Reviewed the telemetry. 8600 entries in the CSV file, 40% with zero, "No signal received". Apparently nothing was done to find and correct the transmission problem. The broken retractor was replaced however.
There is no doubt in my opinion that the fault lies in the ST-16. The "H" sends out several telemetry signals every second regularly. GPS is always solid and correct. I don't think the problem lies in the GPS unit. It is likely in the ST-16.
I've contacted Yuneec again and have yet another Return Authorization. I've become exasperated with Yuneec. Shipping is a week out and a week back. The copter is gone for about a month. I don't believe this particular unit will ever get fixed. I have requested a replacement. I'm not optimistic of that happening!
Apologies for the long post!!
David