Try the following
all faders to 90% maximum (how much is the twitchy amount)
the even faders back to zero, the others 90% so (Fader1=90% fader2=0, Fader3=90%,Fader 4=0 .....)
is the twichy amount now bigger?
On the Brainboard change
rail capacitors (Blocking cap)
C40
C41
C35
C36
Electrlytic Capacitors
C43
C46
C37
costs of the devices about 4-5 Euros (expensive calculation, normally about 2 Euros)
amout of work if yor bring only the brainboard (30minutes ) -> it should not cost more than 50 Euros.
Background (I think either your ref voltage swings or the blocking caps are bad (problems when switching between the fader channels).