I've been flying with the APM for months, and until a week ago, didn't have any troubles.
Last week I had a plane go down in the woods after issuing a guided mode command "fly to here". I wasn't able to determine the cause of the crash, but the APM was ruined after sitting overnight in the rain.
Today, I put a new APM and new electronics and checked all the surfaces, etc. No wind at all. I sent it up with the default config that usually works for it, and these are the tracks. Any advice for figuring out the issue?
I also had another major problem today when I switched into RTL mode and my plane went into a total death spiral. Couldn't recover from it either for some reason...
Replies
http://code.google.com/p/ardupilot-mega/wiki/ConfigFiles
And with my extremely limited experience from looking at your pic it looks like it's snaking to waypoints so some tuning is required (decrease nav roll P from memory)
BUT if you haven't tried the config file on the above page i'd give that a go first.