RTL looses altitude

I am already quite familiar with my APM 2.6 on my Phantom X61. Modes work, Auto flight works, Stabi, Acro, Failsafe etc. do more or less what they should. Only problem: In RTL the plane changes its altitude. It first moves up to some altitude close to the defined (90m) and then slowly drops with each circle. Throttle stays at 75% as set. I have to switch to manual to prevent a crash.
Any idea?

Hey Chris,

I approved the post to get the ball rolling, but please note there are a few details listed in the instructions that you need to include in order to give us the best shot at understanding your problem. Have a read with those please and then provide the missing stuff noted.


Hi Josh, Thanks! See below please:
My Setup:
APM ArduPilot Mega 2.6 - vertical
GPS uBlox LEA-6H with external compass
Firmware: V2.76
Plane: Phantom FX-61 flying wing
Please find the log attached.