I have a Phantom 2 with Lightbridge and a H3-3d. Default Gains except as you suggested ATTI is at 120. Compass is correctly adjusted. All firmware is up to date. RC is a Taranis. I take the Phantom up to 75 or 100 feet with GPS (no less than 8 satellites) hoover for short while then begin to descend straight down at a constant moderately slow speed by holding the throttle in a steady position a little below midpoint. At some point in the descent the drone will essentially go into a free fall. Sometimes if I throw the throttle all the way up and I will get the drone to slow down and begin an ascent. I have swapped the FC from a Phantom Vision Plus thinking I had a FC issue. Same pattern has happened. Issue has also happened with different batteries. Does not seem like an antennae problem as I have replicated the issue with the Phantom in several different orientations. Thank goodness for some very dense unmowed grass as a couple of times the Phantom has dropped fifteen or twenty feet to the ground. I am not sure where to go from here.