I know it was an issue in FSX, but it's gone since some SP. But surprisingly I got it recently with 773 in P3D - a/c in unstable in pitch channel, rocking badly up and down up to the crash. CLB/DES mode are smooth, but on cruise I'm not able to get it keep the alt. All is up to date and patched. ASX on and off is the same, so it's not a WX issue. Any ideas?

 

Rds,

Andrey Osnovsky

Any ideas?

 

Despite the claim that it's not a weather issue, that's still my first thought. Unless your theme is specifically "clear all weather" or whatever P3D calls it, weather is going to be a factor somehow. From there, I'd consider any programs that might be messing with the aircraft's weight: trying to load the plane through third party software like TOPCAT, FSPassengers, or something like that. The next thought is that you have some soft of axis interaction, like a hardware trim axis (though this would more likely show in the AP disconnecting).