Hi @tridge may be the log in the previous post will shed more light on the default PIDs. This is the 3rd instance of someone experiencing the flutter on maiden. So far all 3 where using small flying wings S800 and Wing Wing Z-84
I will halve the default pitch D for the 3.9.2 release. Can you try with defaults for 3.9.1, but with half the PTCH2SRV_D term? The default is 0.08, so try with 0.04.
Cheers, Tridge