I've been flying this F550 for well over a year now and it recently developed a problem with twitchy stability and varying altitude.
You can see all of my previous successful and issue free flight videos on my channel.
So far, everything seems to check out fine (motors, ESCs, GPS, etc.) and I've even tried the black tape mod over the NAZA with no change.
I'm thinking perhaps the internal barometer took a dump, but I'm open to suggestions.