I have been trying the x-plane interface over the last couple of days and could't manage to tune the airframe - it always ended up spiraling in after switching to any of the stabilized modes. This had been working fine before. I noticed a substantial time lag (1 second?) between the aircraft attitude changing in x-plane and the corresponding change in the HUD attitude in the mission planner, so suspected that the delay was making the control loops unstable. It seems like the control surfaces update quickly enough (APM communication to x-plane) - both because I can see them update on the airplane model in x-plane and because it is easy to fly in manual mode. The problem seems to be on the aircraft data return (x-plane to APM). This was with version 1.0.41, but I updated to 1.0.42 and it was the same. X-plane is the latest 9.7. My APM was loaded with the HIL firmware from the mission planner.
I can't be sure which version of the misison planner I was using before when it was working, but found a copy of version 1.0.0 so tried that. All works fine with this version and I can tune it up to a nice crisp response. X-plane wasn't reloaded between the runs with the two different versions, and neither was the APM reset, so I don't believe it is anything in the way I have set the data I/O options in x-plane.
I am using Windows 7 32 bit, but also tried it on a second PC with Windows 7 64 bit. Same result. X-plane update rates look fine (smooth motion) in both cases and have good processors and graphics cards.
Any ideas on what is happening?