To clarify, I think you're saying that during your 16,000' mission the display had some glitches, but never went away, and the HD signal bars for video at the top of the screen were strong until you turned around. So you never lost video, or got a weak HD signal bar, but you turned around (presumably due to battery reserve) and returned. However with GL300C you would lose HD signal bars and camera view at 12,500 often.
What I've noticed is the stock GL300C loses the control signal first. This signal strength is shown at the top using the RC ICON, next to the HD signal bar display. With GL300C, whenever my control signal gets weak I still have good video signal bars showing, but when the control signal is lost, automatically my video link is lost too, even though I had a good HD signal strength bars right up until the control signal disconnected. This behavior is different than my P3P, which NEVER loses control signal with the craft. It always loses video first (my preference) at about 12,000 to 15,000' (with windsurfer), but when that happens I can always ascend in an effort to regain the video signal, which usually works.