Interesting problem here. If I flash my APM 2.5 with the plane firmware, then flip over to the flight data window.. I see my artificial horizon jumping around all over the place once the system is armed. This is while the APM is sitting perfectly still on the ground. However if I flip over to the quad firmware on the same board and arm the board the artificial horizon sits perfectly still. Is this something going on with the hardware or the software? I had one of my planes decided to fall out of the sky from perfectly good stabilized flight. So I'm trying to eliminate things one at a time.
This is now running version 2.68, but was running a previous version of code before flipping back and forth to test. I've also tested on two other APM boards I have. Both of those boards hold perfectly stable artificial horizon.