Confirmed, Aviation Traffic also causes it. Flying along happily, decided to set AI Offline Traffic to 100 on both sliders. Immediately all instrumentation whacked out.
So, the following is now advised:
Options > General > Traffic:
General Aircraft Density to 0
Aviation Aircraft to Off
Obviously, you could set them to 10 or 20 if you want to see if it still breaks anything, but 0 / off is best for a trouble free flight.
I’m now guessing mass multiplayer sessions, like the one yesterday on Squirrel’s stream, will also cause issues. I had a multiplayer session last night but there was about 15 of us spread out and I had no ill effects.
This may explain why the General Aircraft Density fix alone worked for some and not others, because some still had aviation traffic enabled.
I also reckon that when I tested Live Air Traffic and had no ill effects, it was because the sim was not loading it in, it seemed really sparse (I have been having issues with Teredo, might be a factor).
Basically, any other aircraft is a threat.
(To confirm, I was in an A320)
Edit: Bug report updated with these findings.